GB2074678A

GB2074678A - Anti-lock brake system for an automotive vehicle

Info

Publication number: GB2074678A
Application number: GB8111621A
Authority: GB
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1980-04-14
Filing date: 1981-04-14
Publication date: 1981-11-04
Also published as: FR2480215A1; DE3114059A1; JPS56146449A; FR2480215B1; GB2074678B

Abstract

This invention relates to an anti-lock brake system control equipment including a master cylinder comprising a primary piston (14) and, in tandem relationship a secondary piston (16) disposed within a bore (12) of a cylinder housing (10). The secondary piston has an effective area (A2) facing a secondary pressure chamber (20) for rear brakes, which area is smaller than the effective area (A1) of the primary piston facing a primary pressure chamber (18) for front brakes. The hydraulic pressure (P2) in the secondary chamber is equal to that (P1) in the primary chamber until a regulator valve means (144) is opened by pressure (P3) in a chamber (18) exceeding a predetermined value to allow fluid flow from the chamber (18) to reservoir (24). Owing to the difference of the effective areas of the primary piston and the secondary piston, hydraulic pressure to be applied to the rear wheel slave cylinder is then reduced to prevent locking of the rear brakes. <IMAGE>

Description

SPECIFICATION Improvements in and relating to a brake system for an automotive vehicle The present invention relates generally to a hydraulic brake system for an automotive vehicle.

More specifically the invention relates to brake system control equipment including a tandem master cylinder having a primary piston and a secondary piston in tandem relationship for independently supplying working fluid to the front and rear wheel slave cylinders.

A brake system master cylinder which has a primary piston for supplying working fluid to the rear wheel slave cylinders and a secondary piston for supplying the working fluid to the front wheel slave cylinders, is known. In such a master cylinder, the primary piston and the secondary piston are positioned in a housing in tandem relationship so that they can be operated together by operation of the brake pedal. While, as is well known, the braking force applied to the front wheel brakes is greater than that applied to the rear wheel brakes, the phenomenon of nose diving can lead to locking of the rear wheels prior to locking of the front wheels and thus to skidding of the vehicle. The result of locking of the rear wheels prior to locking of the front wheels is that the vehicle tends to spin about the front wheels.

To prevent spinning of the vehicle in this way there have been developed brake systems having proportioning valves in the hydraulic circuit for the rear wheel brake system. This leads to an increase in the cost of the brake system and makes the hydraulic circuit for the brake system so complicated that it is difficult to assemble and maintain.

Therefore, it is an object of the present invention to provide a tandem master cylinder in the brake system which can enable a simplified hydraulic circuit to be used and which can yet effectively prevent the rear wheels from locking.

Another and more specific object of the present invention is to provide a tandem master cylinder having means for relieving working fluid when the hydraulic pressure applied to the front or rear wheel slave cylinder exceeds a predetermined value.

For accomplishing the above-mentioned and other objects a master cylinder according to the present invention comprises a primary piston and a secondary piston disposed within a bore of a cylinder housing in tandem relationship. The effective area of the secondary piston facing a secondary pressure chamber is smaller than the effective area of the primary piston facing a primary pressure chamber. The hydraulic pressure in the secondary chamber is regulated by a regulator valve means working in opposition to that pressure. The regulator valve means becomes operative when the hydraulic pressure in the secondary pressure chamber exceeds a predetermined value.Owing to the difference of the effective areas of the primary piston and the secondary piston, the hydraulic pressure to be applied to the front and rear wheel slave cylinders is varied to reduce the pressure in the rear wheel slave cylinder in relation to the front wheel slave cylinder thereby preventing skidding of the vehicle.

The present invention provides brake system control equipment for an automotive vehicle including a tandem master cylinder, the cylinder comprising: a cylinder housing having a bore which is stepped to form two longitudinal bore sections, the smaller diameter section being adjacent to a closed end of the housing:: a primary piston disposed within the bore and defining with the internal periphery of said bore, a primary fluid chamber, a first fluid passage being formed in the cylinder housing for connecting the primary fluid chamber to a fluid reservoir; a secondary piston disposed within the bore in tandem relationship with the primary piston, a primary pressure chamber being defined between the two pistons for communication with rear wheel slave cylinders of the vehicle, a secondary pressure chamber for communication with front wheel slave cylinders of the vehicle being defined within the smaller diameter section of the bore between the secondary piston and the closed end of the cylinder housing and a secondary fluid chamber being defined between the internal periphery of the cylinder housing and the secondary piston and a second fluid passage being formed in the cylinder housing for connecting the secondary fluid chamber to the fluid reservoir; first and second return springs respectively disposed within the primary and secondary pressure chambers for providing return forces to the primary and secondary pistons; a first and second means for establishing communication respectively between the primary fluid chamber and the primary pressure chamber and the secondary fluid chamber and the' secondary pressure chamber; and a connecting means for connecting the primary piston to a brake pedal and for tramsmitting a brake force applied to the pedal to the primary and secondary pistons; the equipment further including a pressure regulating means for regulating hydraulic pressure when the hydraulic pressure in the secondary fluid chamber exceeds a predetermined value whichh pressure regulating means includes a pressure responsive valve means for permitting fluid flow from the fluid reservoir to the secondary fluid chamber, the valve means being operative to establish communication between the secondary pressure chamber and the fluid reservoir upon the pressure in the secondary pressure chamber exceeding the predetermined value.

The present invention also provides brake system control equipment for an automotive vehicle including a tandem master cylinder the cylinder comprising: a cylinder housing having a bore which is stepped to form two longitudinal bore sections the smaller diameter section being adjacent to a closed end of the housing; a primary piston disposed within the bore and defining, with the internal periphery of said bore, a primary fluid chamber, a first fluid passage being formed in the cylinder housing for connecting the primary fluid chamber to a fluid reservoir;; a secondary piston disposed within the bore in tandem relationship with the primary piston, a primary pressure chamber being defined between the two pistons for communication with rear wheel slave cylinders of the vehicle, a secondary pressure chamber for communication with front wheel slave cylinders of the vehicle being defined within the smaller diameter section of the bore between the secondary piston and the closed end of the cylinder housing and a secondary fluid chamber being defined between the internal periphery of the cylinder housing and the secondary piston and a second fluid passage being formed in the cylinder housing for connecting the secondary fluid chamber to the fluid reservoir; first and second return springs respectively disposed within the primary and secondary pressure chambers for providing return forces to the primary and secondary pistons;; first and second means for establishing communication respectively between the primary fluid chamber and the primary pressure chamber and the secondary fluid chamber and the secondary pressure chamber; and connecting means for connecting the primary piston to a brake pedal and for transmitting a brake force applied to the pedal to the primary and secondary pistons; the equipment further including pressure regulating means for regulating hydraulic pressure when the hydraulic pressure in the secondary fluid chamber exceeds a predetermined value, which pressure regulating means includes a pressure responsive valve means for permitting fluid flow from the fluid reservoir to the secondary fluid chamber, the valve means being arranged also to allow fluid to flow from the secondary fluid chamber to the fluid reservoir when the pressure in the secondary fluid chamber exeeds the predetermined value.

The present invention also provides a brake system incorporating control equipment according to the invention.

Brake system control equipment constructed and arranged to operate in accordance with the invention and a brake system incorporating that equipment will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a longitudinal section of the equipment, and Figure 2 is a graph showing the relationship between the brake pressures respectively applied to the rear wheel slave cylinder and the front wheel slave cylinder of a brake system incorporating the equipment shown in Fig. 1.

Referring to the accompanying drawings, Fig. 1 illustrates control equipment having a tandem master cylinder comprising a cylinder housing 10 formed with a bore 1 2 therein the bore extending along the longitudinal axis of the housing 10. A primary piston 14 and a secondary piston 1 6 are disposed with the bore 1 2 in space apart relationship. Between the primary and secondary pistons 14 and 1 6 there is defined a primary pressure chamber 18. Between the secondary piston 1 6 and the end 22 of the housing 10 there is defined a secondary pressure chamber 20.

A fluid reservoir in the form of a header tank 24 for storing working fluid is connected to the cylinder housing 10. The fluid reservoir 24 is provided with a pair of fluid passages 26 and 28 protruding outwardly from its housing. Although not clearly shown in Fig. 1, the fluid passages 26 and 28 connect, respectively, the primary and secondary pressure chambers 18 and 20 to different chamber portions of the fluid reservoir 24. The passages 26 and 28 are respectively formed at their ends, with flange portions 30 and 32. Fittings 34 and 36 are mounted on the outer circumferences of the passages 26 and 28 and are respectively secured in cylindrical projecting portions 38 and 40 of the cylinder housing -- grooves 42 and 44 in the fittings 34 and 36 engaging with inwardly projecting flanges 46 and 48 on the portions 38 and 40.The fluid reservoir 24 communicates with the bore 1 2 in the cylinder housing 10 via inlet ports 50 and 52 formed in the cylinder housing 10. Also, the fluid reservoir 24 communicates with the cylinder bore 1 2 via relief ports 54 and 56 formed in the cylinder housing.

The primary piston 14 is formed on its outer circumference with an annular recess 58 to define a primary fluid chamber 60 with the internal periphery of the cylinder housing. The primary piston 14 is thus formed with a pair of flange sections 62 and 64 at each of its ends.

Likewise, the secondary piston 1 6 is formed on its outer circumference with annular recess 66 resulting in a pair of flange sections 68 and 70 at each of its ends. The annular recess 66 defines a secondary fluid chamber 72 with the internal periphery of the cylinder housing 10.

On the outer circumference of the flange section 64 of the primary piston 14, there is formed an annular recess 74 for receiving a sealing ring 76. The primary piston 14 is also formed with a longitudinally extending recess 78 at the end adjacent the flange section 64. An input rod 80 for transmitting the brake force applied to a brake pedal (not shown) is received within the recess 78. On its other end remote from the input rod 80, the primary piston 1 4 has a projecting portion 82 extending along the central axis thereof. At the centre of the projecting portion, a threaded bore 84 is formed. A plurality of longitudinal through-going openings 85 are formed in the flange section 62 adjacent to its circumference.An annular piston cup 86 is mounted in the primary pressure chamber 1 8 and adjacent to the openings 85 so that it permits fluid to flow in a direction from the primary fluid chamber 60 to the primary pressure chamber 1 8 but to restrict fluid flow in the opposite direction.

A substantially disc-shaped spring seat 88 is mounted on the projecting portion 82 of the primary piston 14. Spaced from the spring seat 88 is another spring seat 90. The spring seat 90 has a cup-shaped central portion 92 and circumferential flange portion 94. The bottom of the central portion 92 is formed with an opening 96. A fastening screw 98 passes through the opening 96 and engages with the threaded bore 84 in the projecting portion 82 via a central opening 100 formed in the spring seat 88. Between the spring seats 88 and 90, a set spring 102 is disposed to urge apart the spring seat 90 and the primary piston 14. The flange portion 94 of the spring seat 90 engages an inwardly extending stop 104 screwed into the cylinder housing 1 0.

The flange sections 68 and 70 of the secondary piston 1 6 have different diameters. The flange section 68 has a smaller diameter than that of the flange section 70 and is located with a smaller diameter section 106 of the bore 1 2. The flange section 70 is formed with an annular recess 108 in its outer circumference to receive a sealing ring 11 0. The secondary piston 1 6 also has a projecting portion 11 2 extending longitudinally from the end of that piston facing the secondary pressure chamber 20. The flange section 68 is formed with a plurality of longitudinal through-going openings 114 adjacent its outer circumference.An annular piston cup 11 6 is mounted in the chamber 20 adjacent to the openings 114 so that it permits fluid to flow in a direction from the secondary fluid chamber 72 to the secondary pressure chamber 20 but to restrict fluid flow in the opposite direction. Adjacent to the piston cup 116, the projecting portion 11 2 is formed with an annular recess 11 on its outer circumference.A spring seat 1 20 is received within the annular recess and is located opposite to the piston cup 11 6. One end of a set spring 1 22 is seated on the spring seat 1 20 and its other end is seated on the closed end of the bore 1 2. Thus, the set spring 1 22 exerts a force on the secondary piston 1 6 in a direction towards the primary pressure chamber 1 8. Since the secondary piston 1 6 abuts against the flange portion 94 of the spring seat 90 and movement of the spring seat 90 in a direction towards the primary pressure chamber 1 8 is limited by the stop 104 movement of the secondary piston 1 6 in that direction is also limited.

As shown in Fig. 1, a valve housing 1 30 is secured inside the fluid reservoir 24 in the region of the fluid passage 28. The valve housing is formed with a slot 1 32 which extends aroung a major part of the periphery of the housing. A duckbill-shaped valve member 1 34 is disposed within the valve housing 1 30. The open end 1 36 of the valve member 1 34 faces towards the fluid reservoir 24. The valve member 1 34 permits fluid to flow in a direction from the fluid reservoir 24 to the cylinder bore 1 2 via the inlet port 52 (as indicated by the arrow in Fig. 1).

The valve member 1 34 is seated on an annular valve seat 1 38 which is fitted on the internal periphery of the fluid passage 28 at its end remote from the valve housing 1 30. As seen from Fig. 1, the outer periphery of the valve member 1 34 is spaced from the internal periphery of the fluid passage 28 to define a chamber 140. A spring 1 42 is also disposed within the valve housing 1 30 one end of it being seated within the valve member 1 34. Thus, the valve member 134, the spring 142 and the chamber 140 constitute a pressure difference responsive valve 144.

The cylinder housing 10 is further formed with primary and secondary ports 1 50 and 1 52 respectively communicating with rear and front wheel slave cylinders (not shown) via fluid passages (not shown).

In operation, when the brake pedal is in its released position, the positions of the primary piston 14 and the secondary piston 1 6 with respect to the cylinder housing 10 are as shown in Fig. 1. In this condition, the primary fluid chamber 60 communicates with the fluid reservoir 24 via the passage 26 and the inlet port 50. Likewise, the secondary fluid chamber 72 communicates with the fluid reservoir 24 via the pressure difference responsive valve 144 and the inlet port 52. On the other hand, the primary pressure chamber 1 8 communicates with the fluid reservoir 24 via the relief port 54 and the secondary pressure chamber 20 communicates with the fluid reservoir 24 via the relief port 56.

Upon depressing the brake pedal, the brake pressure is applied to the primary piston 1 4 via the input rod 80 and the primary piston 1 4 is moved towards the primary pressure chamber 1 8.

The primary piston 14 interrupts communication between the primary pressure chamber 1 8 and the fluid reservoir 24 via the relief port 54 during its motion towards the primary pressure chamber, the piston cup 86 closing the relief port 54. Further travel ofthe primary piston 14, results in the fluid pressure in the primary piston chamber being increased to force the working fluid in that chamber to flow through the primary port 1 50 and into a fluid passage leading to the rear wheel slave cylinder. The working fluid applied to the rear wheel slave cylinder urges apart the pistons in that cylinder. As a result, the brake lining engages the brake drum as is well knows.

Corresponding to the motion of the primary piston 14 toward the primary pressure chamber 18, the secondary piston 1 6 is urged toward the secondary pressure chamber 20 by the applied brake pressure being transmitted via the screw 98 and the spring 102 and the spring seat 92.

Thus, the secondary piston 1 6 is moved toward the secondary pressure chamber 20. During its travel towards the secondary pressure chamber 20, the piston cup 11 6 closes the relief valve 56 to disconnect the secondary pressure chamber 20 from the fluid reservoir 24. At the same time, as it moves toward the secondary pressure chamber 20, the flange section 68 slides along the internal periphery of the section 106 to reduce the volume of the secondary fluid chamber 72. By reducing that volume the fluid pressure in the secondary fluid chamber 72 is increased to force the working fluid in that chamber to flow toward the secondary pressure chamber 20 through the openings 114 and the piston cup 116.The fluid pressure in the scondary fluid chamber 72, deforms the piston cup 11 6 so that a gap is formed between its outer periphery and the inner periphery of the bore 1 2. The fluid in the secondary fluid chamber 72 flows through that gap. At the same time, the fluid pressure in the secondary pressure chamber 20 is increased by reduction of the volume of the secondary pressure chamber 20. The fluid flowing into the pressure chamber 20 from the secondary fluid chamber 72 and the reduction in volume of the secondary pressure chamber 20 results in an increase in the hydraulic pressure in the secondary pressure chamber 20.The fluid thus pressurized is fed to the front wheel slave cylinders through the secondary port 1 52. The pressurized fluid supplied from the master cylinder through the secondary port 1 52 actuates the front wheel slave cylinders so that the brake lining engages the brake drum.

Since the working fluid in the secondary pressure chamber 20 is pressurized not only by reduction of its volume but also by fluid flowing into it from the secondary fluid chamber 72, sufficient brake pressure for the front wheel slave cylinders is obtained by a relatively short stroke of the secondary piston 1 6 and as a result by a relatively short stroke of the brake pedal.

This may lead to better brake feel.

By further application of the brake pedal, the primary and secondary pistons 14 and 1 6 move further into the primary and secondary pressure chambers 1 8 and 20. Since the primary and secondary pressure chambers 1 8 and 20 are respectively disconnected from the fluid reservoir 24 the fluid pressure in the primary and secondary pressure chambers 1 8 and 20 and, in turn, that applied to the rear and front wheel slave cylinders is increased by the motion of the primary and secondary pistons 14 and 1 6. By increasing the fluid pressure in the wheel slave cylinders the pressure applied to the brake linings is also increased.

If the fluid pressure in the secondary fluid chamber exceeds a predetermined set value the valve member 1 34 of the pressure difference responsive valve 144 is forced off the valve seat 1 38 against the fluid pressure in the chamber 140. Consequently, the pressurized fluid in the secondary fluid chamber 72 is permitted to flow into the fluid reservoir 24. At the same time, the pressure of the fluid accumulated in the secondary pressure chamber 20 fprces the piston cup 11 6 against the end of the fluid passage 114 to prevent fluid flow from the secondary fluid chamber 72 to the secondary pressure chamber 20. Thereby, the fluid pressure in the secondary pressure chamber 20 and, in turn, that applied to the front wheel slave cylinders is maintained at the set pressure.

By considering the forces acting on the secondary piston 11 6 (omitting the forces exerted by the springs 102 and 122) it can be seen that the fluid pressures respectively in the primary and secondary pressure chambers 18 and 20 in the balanced condition can be represented by the equation: A1Pa = A2P2 + (A1 - A2)P3 where A, = effective area of the primary pressure chamber A2 = effective area of the secondary pressure chamber; P, = fluid pressure in the primary pressure chamber; P2 = fluid pressure in the secondary pressure chamber; P3 = fluid pressure in the secondary fluid chamber.

Therefore, the hydraulic pressure applied to the front wheel slave cylinders can be represented by: A, A1 -A2 P2 = P. - P3 A2 A2 As will be apparent from the above equation, since the fluid pressure applied to the secondary pressure chamber is gradually increased until the accumulated pressure reaches the set pressure of the pressure responsive valve 144 and thereafter become constant, the difference in the hydraulic pressure applied to the front and rear wheels slave cylinders is determined by the difference in the effective areas of the primary and secondary pressure chambers 18 and 20 and the set pressure of the pressure difference responsive valve 144.Thus, the ratio of the hydraulic pressure in the front and rear wheel slave cylinders can be determined by determining the difference of the effective areas of the primary and secondary pressure chambers 1 8 and 20. As shown in Fig. 2, as the hydraulic pressure applied to the rear wheel slave cylinders is varied the increasing ratio thereof at a branch point f, namely, the hydraulic pressure to be applied to the rear wheel cylinders is increased linearly within a range from a point e to the branch point fand thereafter further increased in the reduced ratio in a range from the branch point fto the point g linearly. Within the range e to f, the increasing ration of the hydraulic pressure for the rear wheel cylinders is same as that for the front wheel cylinders. The branch point fis determined by the set pressure of the pressure difference responsive valve 144.

When the braking force applied to the brake pedal is released, the primary and secondary pistons 14 and 1 6 are forced toward their neutral positions by the return springs 102 and 1 22.

At this time, owing to the pressure difference between the primary pressure chamber 1 8 and the primary fluid chamber 60 and the secondary pressure chamber 20 and the secondary fluid chamber 72 the fluid respectively in the primary and secondary fluid chambers 60 and 72 flows into the primary and secondary pressure chambers 1 8 and 20 through gaps respectively formed between the internal periphery of the bore 1 2 and the outer periphery of the piston cups 86 and 11 6 through the passages 85 and 114.During the travel of the secondary piston by expanding of the volume of the secondary pressure chamber, the pressure difference between the fluid pressure 24 and the secondary fluid chamber 72 causes opening of the normally closed end of the duckbill valve member 1 34 to permit fluid flow through it.

The manner in which the fluid pressures in the front and rear slave cylinders vary as the brake pedal is depressed can readily be seen and understood from Fig. 2 and the equation set out above, since P2 is effectively equal to the fluid pressure in the front slave cylinder and P, is effectively equal to the fluid pressure in the rear slave cylinders.

Until the pressure in the secondary fluid chamber 72 reaches the pressure at which the valve 144 opens (that valve being responsive to the difference between the pressure in the secondary fluid chamber 72 and the pressure in the fluid reservoir 24) the pressures P, and P2 are effectively equal to one another, which implies that the pressure P3 is effectively equal to P2.

When P3 reaches the predetermined value at which the valve 144 opens, it ceases to increase as P2 rises further with further depression of the brake pedal. Thus, P3 becomes less than P2 but fluid is prevented from flowing from the secondary pressure chamber 20 to the secondary fluid chamber 72 by the piston cup 116. Accordingly, instead of the equation set out reducing 'to P, = P2, A2 (A1-A2) P1 = ----- . P2 + P3 ' , where P3 is now A, A, a constant. Since A2 is less than A1, the slope of the graph of P, plotted against P2 becomes less than 1 as shown in Fig. 2.

After the so-called branch point f has been reached, P, and P2 each continue to increase with further depression of the brake pedal, but P1, increases more slowly than P2 so that the value of P2 exceeds the value of P1 by an amount that increases as the brakes are applied harder.

The position of the point fin Fig. 2 depends on the pressure difference at which the valve 144 is set to open while the slope of the curve beyond that point depends on the ratio of A2 to A1.

Claims

1. Brake system control equipment for an automotive vehicle including a tandem master cylinder, the cylinder comprising: a cylinder housing having a bore which is stepped to form two longitudinal bore sections, the smaller diameter section being adjacent to a closed end of the housing; a primary piston disposed within the bore and defining, with the internal periphery of said bore, a primary fluid chamber a first fluid passage being formed in the cylinder housing for connecting the primary fluid chamber to a fluid reservoir;; a secondary piston disposed within the bore in tandem relationship with the primary piston, a primary pressure chamber being defined between the two pistons for communication with rear wheel slave cylinders of the vehicle, a secondary pressure chamber for communication with front wheel slave cylinders of the vehicle being defined within the smaller diameter section of the bore between the secondary piston and the closed end of the cylinder housing and a secondary fluid chamber being defined between the internal periphery of the cylinder housing and the secondary piston and a second fluid passage being formed in the cylinder housing for connecting the secondary fluid chamber to the fluid reservoir;; first and second return springs respectively disposed within the primary and secondary pressure chambers for providing return forces to the primary and secondary pistons; a first and second means for establishing communication respectively between the primary fluid chamber and the primary pressure chamber and the secondary fluid chamber and the secondary pressure chamber; and a connecting means for connecting the primary piston to a brake pedal and for transmitting a brake force applied to the pedal to the primary and secondary pistons; the equipment further including a pressure regulating means for regulating hydraulic pressure when the hydraulic pressure in the secondary fluid chamber exceeds a predetermined value, which pressure regulating means includes a pressure responsive valve means for permitting fluid flow from the fluid reservoir to the secondary fluid chamber, the valve means being operative to establish communication between the secondary pressure chamber and the fluid reservoir upon the pressure in the secondary pressure chamber exceeding the predetermined value.

2. Brake system control equipment for an automotive vehicle including a tandem master cylinder, the cylinder comprising: a cylinder housing having a bore which is stepped to form two longitudinal bore sections, the smaller diameter section being adjacent to a closed end of the housing; a primary piston disposed within the bore and defining, with the internal periphery of said bore a primary fluid chamber a first fluid passage being formed in the cylinder housing for connecting the primary fluid chamber to a fluid reservoir;; a secondary piston disposed within the bore in tandem relationship with the primary piston, a primary pressure chamber being defined between the two pistons for communication with rear wheel slave cylinders of the vehicle, a secondary pressure chamber for communication with frorrt wheel slave cylinders of the vehicle being defined within the smaller diameter section of the bore between the secondary piston and the closed end of the cylinder housing and a secondary fluid chamber being defined between the internal periphery of the cylinder housing and the secondary piston and a second fluid passage being formed in the cylinder housing for connecting the secondary fluid chamber to the fluid reservoir;; first and second return springs respectively disposed within the primary and secondary pressure chambers for providing return forces to the primary and secondary pistons; first and second means for establishing communication respectively between the primary fluid chamber and the primary chamber and the secondary fluid chamber and the secondary pressure chamber; and connecting means for connecting the primary piston to a brake pedal and for transmitting a brake force applied to the pedal to the primary and secondary pistons; the equipment further including pressure regulating means for regulating hydraulic pressure when the hydraulic pressure in the secondary fluid chamber exceeds a predetermined value, which pressure regulating means includes a pressure responsive valve means for permitting fluid flow from the fluid reservoir to the secondary fluid chamber, the valve means being arranged also to allow fluid to flow from the secondary fluid chamber to the fluid reservoir when the pressure in the secondary fluid chamber exceeds the predetermined value.

3. Brake control equipment equipment for an automotive vehicle including a tandem master cylinder, the cylinder comprising: a cylinder housing having a bore which is stepped to form two longitudinal bore sections, the smaller diameter bore section being adjacent to a closed end of the housing; a primary piston for cooperation with a brake pedal and being disposed within the larger diameter section of the bore and defining, with the internal periphery of the bore, a primary fluid chamber a first fluid passage being formed in the cylinder housing for connecting the primary fluid chamber to a fluid reservoir;; a secondary piston disposed within the smaller diameter section of the bore in tandem relationship with the primary piston and operatively connected to the primary piston, a primary pressure chamber being defined between the two pistons for communication with rear wheel slave cylinders of the vehicle, and a secondary pressure chamber being defined between the closed end of the housing and the secondary piston for communication with front wheel slave cylinders of the vehicle, and a secondary fluid chamber being defined between the internal periphery of the bore and the secondary piston, a second fluid passage being formed in the cylinder housing for connecting the secondary fluid chamber to the fluid reservoir;; first and second return springs respectively disposed within the primary and secondary pressure chambers for providing an initial set force to the primary and secondary pistons; and a first and second means for establishing communication respectively between the primary pressure chamber and the primary fluid chamber and the secondary chamber and the secondary fluid chamber, the first and second means including a respective valve member permitting fluid flow from the fluid chambers to the pressure chamber; the equipment further including a pressure regulating means for regulating hydraulic pressure when the hydraulic pressure in the secondary fluid chamber exceeds a predetermined value, which pressure regulating means includes a pressure responsive valve means permitting fluid flow from the fluid reservoir to the secondary fluid chamber, the valve means being operative to establish communication between the secondary pressure chamber and the fluid reservoir when the pressure in the secondary pressure chamber exceeds the predetermined value.

4. Brake control equipment for an automotive vehicle including a tandem master cylinder, the cylinder comprising: a cylinder housing having a bore which is stepped to form two longitudinal bore sections, the smaller diameter bore section being adjacent to a closed end of the housing; a primary piston for cooperation with a brake pedal and being disposed within the larger diameter section of the bore and defining, with the internal periphery of the bore, a primary fluid chamber a first fluid passage being formed in the cylinder housing for connecting the primary fluid chamber to a fluid reservoir;; a secondary piston disposed within the smaller diameter section of the bore in tandem relationship with the primary piston and operatively connected to the primary piston, a primary pressure chamber being defined between the two pistons for communication with rear wheel slave cylinders of the vehicle, and a secondary pressure chamber being defined between the closed end of the housing and the secondary piston for communication with front wheel slave cylinders of the vehicle, and a secondary fluid chamber being defined between the internal periphery of the bore and the secondary piston, a second fluid passage being formed in the cylinder housing for connecting the secondary fluid chamber to the fluid reservoir;; first and second return springs respectively disposed within the primary and secondary pressure chambers for providing an initial set force to the primary and secondary pistons; and first and second means for establishing communication respectively between the primary pressure chamber and the primary fluid chamber and the secondary chamber and the secondary fluid chamber, the first and second means including a respective valve member permitting fluid flow from the fluid chamber to the pressure chamber; the equipment further including pressure regulating means for regulating hydraulic pressure when the hydraulic pressure in the secondary fluid chamber exceeds a predetermined value, which pressure regulating means includes a pressure responsive valve means for permitting fluid flow from the fluid reservoir to the secondary fluid chamber, the valve means being arranged also to allow fluid to flow from the secondary fluid chamber to the fluid reservoir when the pressure in the secondary fluid chamber exceeds the predetermined value.

5. Brake equipment as claimed in any one of claims 1 to 4, wherein there is provided nonreturn valve means arranged substantially to prevent fluid flowing from the secondary pressure chamber to the secondary fluid chamber other than through the fluid reservoir.

6. Brake equipment as claimed in claim 5, wherein the arrangement is such that, after the said predetermined pressure difference between the fluid pressure in the secondary fluid chamber and the fluid reservoir has been reached, the pressure of the fluid in the secondary fluid chamber remains substantially constant as the pressure of the fluid in the secondary pressure chamber rises further.

7. Brake equipment as claimed in any one of claims 1 to 6, wherein the secondary piston has first and second end sections having different areas, the first end section partly defining the secondary pressure chamber and the second end section partly defining the primary pressure chamber, the first end section having a diameter smaller than the diameter of the second end section.

8. Brake equipment as claimed in claim 7, wherein the first end section of the secondary piston is arranged to take up a position at the end of the smaller diameter bore section remote from the closed end of the housing when the brake is not applied.

9. Brake equipment as claimed in any of claims 1 to 8, wherein the pressure regulating means comprises a duckbill valve normally permitting fluid flow in a direction from the fluid reservoir to the secondary pressure chamber and permitting fluid flow from the secondary fluid chamber to the fluid reservoir when the hydraulic pressure in the secondary pressure chamber exceeds the predetermined value.

10. Brake equipment as claimed in any one of claims 1 to 8, wherein the pressure regulating means comprises a duckbill valve normally permitting fluid flow in a direction from the fluid reservoir to the secondary fluid chamber and permitting fluid flow from the secondary fluid chamber to the fluid reservoir when the hydraulic pressure in the secondary fluid chamber exceeds the predetermined value.

11. Brake equipment as claimed in any one of claims 1 to 10, in which the difference in area of the first and second end sections of the secondary piston enables different hydraulic pressures respectively to a be applied to the front and rear wheel slave cylinders and operation of the pressure regulating means enables the relationship between those hydraulic pressures to be varied.

1 2. Brake equipment as claimed in any one of claim 1 to 11, in which the equipment further includes a fluid reservoir, the first and second fluid passages being connected to the fluid reservoir and the pressure regulating means being disposed between the secondary fluid chamber and the reservoir.

1 3. Brake system control equipment substantially as hereinbefore described with reference to, and as illustrated by the accompaning drawings.

14. A brake system including brake control equipment as claimed in claim 12, and respective slave cylinders connected to the primary and secondary pressure chambers.

1 5. A brake system substantially as hereinbefore described with reference to, and as illustrated by the accompanying drawings.

16. An automotive vehicle which includes a brake system as claimed in claim 14 or claim 15.