GB2053375A

GB2053375A - Vacuum System

Info

Publication number: GB2053375A
Application number: GB8019067A
Authority: GB
Original assignee: Pierburg GmbH
Current assignee: Pierburg GmbH
Priority date: 1979-06-15
Filing date: 1980-06-11
Publication date: 1981-02-04
Also published as: IT1148260B; GB2053375B; FR2459390A1; IT8048951A0

Abstract

In the operation of a motor vehicle vacuum system e.g. vacuum servo operated brakes, the availability of an adequate vacuum at short notice, particularly when pressure rises in the working chamber when the suction requirements exceed the capacity of the suction pump, is achieved by the provision of a vacuum store chamber 3 which is evacuated simultaneously with the working chamber 2, and which is applied to the working chamber 2 when the pressure rises therein by operation of a non return valve 5. The vacuum is applied in a first stage to the working chamber, and in a second stage simultaneously to the working chamber and the vacuum store chamber by a gas control means, which may be a throttle 4 (Figure 1) or a valve (Figure 3, not shown) operating in parallel with the non return valve 5. In a modification of Figure 1 the vacuum pump 1 may be a pressure pump, and the non return valve 5 is reversed to actuate a pressure system instead of a vacuum system. <IMAGE>

Description

SPECIFICATION Operation of a Motor Vehicle Vacuum System This invention relates to a method of operating a motor vehicle vacuum system particulariy for vacuum servo-actuated brakes, comprising a user provided with a working chamber for effecting a vacuum-operated action, and a suction pump for evacuating said working chamber. The invention also relates to arrangements for operating such vacuum systems, and to control valves for use in such systems.

Motor vehicles are nowadays equipped with suction consumers of various kinds, for example vacuum servo-actuated brakes, servo-assisted steering gears and pneumatic suspension systems. The partial vacuum, or suction, is supplied by an air pump driven by the engine of the vehicle. An adequate vacuum must be promptly available within a few seconds under conditions when the suction requirements exceed the capacity of the suction pump, even after the vehicle has been left standing for a long period, to ensure prompt operation of the consumer. The known devices used for supplying suction do not fulfil this requirement fully, i.e. they do not supply the necessary suction quickly enough.

The invention provides a method for operating a motor vehicle vacuum system comprising a user provided with a working chamber for effecting a vacuum-operated action, and a suction pump for evacuating said working chamber; characterised in that in a first stage the said working chamber is evacuated by the said suction pump to a degree of vacuum which is sufficient for a single vacuumoperated action of the user, and in a further stage the said working chamber and a vacuum store chamber are evacuated simultaneously by the said suction pump to the maximum degree of vacuum capable of being effected by the said suction pump, and valve means for applying vacuum in the said vacuum store chamber to the said working chamber when the pressure in the said working chamber exceeds that in the said vacuum store chamber.

The method of the invention is particularly characterised in that the said first and further stages of evacuation are regulated by gas flow control means between the said working chamber and the said vacuum store chamber and connected also to the said suction pump, the said gas flow control means being disposed in parallel with a non-return valve constituting the said valve means which non-return valve is closed during the said first and second stages of evacuation and which is opened when the suction requirements of the said user exceeds the capacity of the said suction pump so that the pressure in the working chamber exceeds that in the vacuum store chamber, whereby the said working chamber is evacuated by the said vacuum store chamber.

In one embodiment of the method of the invention the said gas flow control means is a throttle i.e. a constricted orifice disposed so that action of the suction pump evacuates gas in the said first stage simultaneously from the said vacuum chamber and the said working chamber, the gas being evacuated from the said vacuum chamber flowing at a slower rate therefrom than from the said working chamber.

In a further embodiment of the method of the invention, the said gas flow control means is a control valve adapted to open at a predetermined degree of vacuum, whereby when the said control valve is closed, operation of the said suction pump evacuates the said working chamber in the said first stage, and the said working chamber and the said vacuum store chamber are evacuated simultaneously in the said further stage with the said control valve open.

The invention also provides arrangements for carrying out the methods for operating a vehicle vacuum system as hereinbefore set forth, comprising a user provided with a working chamber for effecting a vacuum-operated action, a suction pump for evacuating the said working chamber to effect a single vacuum-operated action of the user, a vacuum store chamber connected to the said pump whereby the said vacuum store chamber may be evacuated simultaneously with the said working chamber to the maximum degree of vacuum capable of being effected by the said pump, and valve means for applying the vacuum generated in the said vacuum store chamber when pressure in the said working chamber exceeds that in the said vacuum store chamber.

In such an arrangement gas flow control means may be provided to control the flow of gas during evacuation by the suction pump of the working chamber and the vacuum store chamber.

As hereinbefore set forth such gas flow control means may be a throttle, such an arrangement comprising the said user provided with a working chamber and a vacuum store chamber connected in parallel to the said suction pump, and a throttle disposed between the said working chamber and vacuum store chamber to restrict the flow of gas therebetween compared with the rate of flow of gas from the said working chamber during operation of the said suction pump, the said throttle being disposed in parallel with a nonreturn valve adapted to be maintained in a closed position until the pressure in the said working chamber exceeds the pressure in the said vacuum store chamber.

In particular the said throttle and the said nonreturn valve may be built into the said vacuum store chamber, thus providing a compact unit for incorporation into the arrangement.

Such an arrangement may be modified if the user operates by pressure rather than suction. In such circumstances a pressure pump is used instead of a suction pump, and the said nonreturn valve is reversed so as to be adapted to be maintained in a closed position until the pressure in the said working chamber drops to a value less than the pressure in the vacuum store chamber.

In a further embodiment of the arrangement the gas flow control means is a control valve adapted to connect and disconnect the suction pump from the vacuum store chambers Such an arrangement comprises the said user provided with a working chamber and a vacuum store chamber connected in parallel to the said suction pump, and a first non-return valve disposed between the said working chamber and vacuum store chamber adapted to be maintained in a closed position until the pressure in the said working chamber exceeds the pressure in the said vacuum store chamber, the said non-return valve being disposed in parallel with a control valve between the said vacuum store chamber and the said pump and adapted to connect and disconnect the said suction pump and the said vacuum store chamber, and a further non-return valve disposed between the said control valve and the said working chamber, the said further control valve being adapted to be maintained in a closed position when the said control valve is open.

In such an arrangement two suction pumps may be provided between the said working chamber and the said vacuum store chamber whereby when both pumps are evacuating the working chamber, gases being evacuated may be discharged to the atmosphere. Thus in such an arrangement there is disposed between the said working chamber and the said vacuum store chamber a further suction pump in parallel with the said first non-return valve adapted to discharge gases being evacuated from the said working chamber by operation of the said both suction pumps when the said control valve is closed.

A preferred form of the said control valve comprises a housing, a first connection for a suction pump of the arrangement, a second connection for the working chamber of the arrangement and a third connection for the vacuum store of the arrangement, a valve interposed between the said second and third connections and having a diaphragm which is acted on on one face by atmospheric pressure and on its other face by the combined effect of a biasing spring and a vacuum generated by operation of the suction pump of the arrangement, the said diaphragm having an actuator pin which opens the said valve by thrust of its valve shaft inwards against the influence of a further valve spring.

Examples of arrangements and control valves according to the invention are hereinafter described and illustrated in the accompanying drawings, of which: Figure 1 is a schematic representation of an arrangement in which the gas flow control means is a throttle; Figure 2 is a vertical cross-section of a vacuum store chamber which may be utilised in the arrangement of Figure 1 in which the throttle and the non-return valve are incorporated in a wall of the said chamber; Figure 3 is a schematic representation of an arrangement in which the gas flow control means is a control valve; Figure 4 is a schematic representation of an arrangement using a control valve with two suction pumps; Figure 5 is a diagram showing the development of suction in the working chamber of suction consumer devices and in an associated vacuum chamber, with the passing of time;; Figure 6 is a section of a control valve for use with the arrangement of Figure 3; Figure 7 is a diagrammatic representation of the arrangement of Figure 4 applied to a servoassisted brake, and Figures 8 and 9 are diagrammatic representations of other embodiments of control valves.

Referring to Figure 1, a suction pump 1 evacuates a vacuum-operated working chamber 2 e.g. of a servo-assisted brake, through line 6, the pump evacuating but more slowly, a vacuum store chamber 3 through a branch line 7. Line 8 leading to the vacuum store chamber 3 contains connected in parallel with each other, a throttle in the form of a constricted orifice 4 and a nonreturn valve 5 which blocks airflow from the vacuum store chamber back towards the pump.

The throttle 4 ensures that the pump evacuates the working chamber 2 quicker than the vacuum store chamber 3, the working chamber 2 being considerably smaller than the vacuum store chamber 3. Consequently the pump is able to evacuate the working chamber 2 rapidly down to a pressure sufficiently low e.g. to ensure good response of a servo-assisted brake when actuated by an operating pedal (not shown).

The working chamber 2 and a little later the vacuum store chamber 3, are brought down to a pressure corresponding to the greatest suction it is able to produce. If the working chamber 2 consumes suction excessively so that the pump can no longer maintain adequate suction on the working chamber and the pressure in the working chamber rises above that in the vacuum store chamber 3, the non-return valve 5 opens allowing the vacuum store chamber 3 to evacuate the working chamber 2.

The vacuum store chamber 5 shown in Figure 2 has built into it a connection 12 for a suction line, a throttle in the form of a constricted orifice 7 and a non-return valve 8. Such a construction has the advantage of simplicity when incorporated for instance in the arrangement of Figure 1.

In the embodiment shown in Figure 3, one suction pump is provided for evacuating the working chamber 3 of a suction consumer. A comparatively large vacuum store chamber 4 is provided. The chambers 3 and 4 are connected in parallel to a suction pump 2, the vacuum store chamber being connected to the pump by line 17, control valve 8 and line 16.

The function of the control valve 8 is to permit the pump 2 to evacuate the vacuum store chamber 4 but only after the working chamber 3 has been evacuated sufficiently, the non-return valve 6 opening only when the pressure in the working chamber 3 has become greater than the pressure in the vacuum store chamber 4. The valve 6 then opens to aliow the vacuum store chamber 4 to evacuate the working chamber 3.

The arrangement functions as follows. It will be assumed that before the pump is activated all parts of the system have atmospheric pressure and control valve 8 is closed. When the pump 2 is started up, it first evacuates the suction user (working chamber 3) up to the switching suction P)6). This is the degree of vacuum in the working chamber 3 at which the control valve opens connecting the pump through lines 1 6 and 17 to the vacuum store chamber 4. The suction in the working chamber 3 remains at the switching suction P,s) until the vacuum store chamber 4 has been evacuated up to the same suction Pls). The vacuum store chamber 4 and the working chamber 3 are then evacuated together up to the highest suction the pump 2 is capable of providing.A rise in pressure in the working chamber 3 to above the pressure in vacuum store chamber 4 is abruptly removed by the opening of non-return valve 6.

In the embodiment shown in Figure 4, two suction pumps are provided for evacuating the working chamber 3 of a suction user. A comparatively large vacuum store chamber 4 is provided. The chambers 3 and 4 are connected in parallel to a suction pump 2, the vacuum store chamber being connected to the pump by line 17, control valve 8 and line 16, the working chamber being connected to the pump 2 by line 12, nonreturn valve 7 and line 11. Between the said working and vacuum store chambers in line 4 is provided in parallel with each other a second suction pump 1 and non-return valve 6. A further non-return valve 5 is provided in discharge line 15.

The function of control valve 8 is to permit the pump 2 to evacuate the vacuum store chamber 4, but only after the working chamber 3 has been evacuated sufficiently. The non-return valve 6 located in by-pass 9 around the pump 1, is adapted to open only when the pressure in the working chamber is in excess of that in the vacuum store chamber 4. The valve 6 then opens and the excess pressure in the working chamber is then abruptly discharged by the vacuum in vacuum store chamber 4.

The arrangement functions as follows. As in Figure 3 it will be assumed that before the pumps are activated all parts of the system have atmospheric pressure and control valve 8 is closed. When the two pumps are started up, at first they both evacuate the working chamber 3 through lines 10,11, 12 and 13, the pump 1 discharging to the atmosphere through lines 14 and 1 5 and through the non-return valve 5. This continues until the degree of vacuum in the working chamber has been brought up to a switching suction P(s,. This is the degree of vacuum at which the control valve 8 opens, connecting pump 2 through lines 16 and 1 7 to the vacuum store chamber 4.As the pressure in vacuum store chamber 4 is at this instant at atmospheric pressure, when the control valve 8 opens non-return valve 7 closes and with evacuation of the working chamber 4, non-return valve 5 also closes. Consequently, while pump 2 is evacuating the vacuum store chamber 4, the working chamber 3 is being evacuated by pump 1 acting in series witfi pump 2. Evacuation of vacuum store chamber 4 continues up to the greatest suction which pump 2 is capable of providing. Thus during operation of a motor vehicle incorporating a vacuum-operated brake incorporating the arrangement if at any time the absolute pressure in the working chamber 3 exceeds the pressure in vacuum store chamber 4, the non-return valve 6 opens and vacuum store chamber 4 abruptly evacuates the working chamber 3.If the degree of vacuum in working chamber 3 decreases so greatly that it becomes less than the switching suction Pls), the control valve 8 closes and the two pumps, working in parallel with each other, evacuate the working chamber 3 back to switching suction Pls).

Figure 5 illustrates the behaviour of the two degree of vacuums in the vacuum store chamber 4 and in the working chamber 3 of the arrangement of Figure 3, i.e. an arrangement utilising a control valve. In this graph suction is measured on the ordinate and time on the abscissa. It is assumed that when the pump 2 is started up, suction is everywhere zero, as indicated at Plo) in the lower left corner of the graph. If the pump 2 were to draw air only from the working chamber 3, the suction in the working chamber 3 would follow the curve 3a at the left in the graph, the suction increasing towards the greatest suction (start) which the pump 2 is capable of delivering.However if the pump 2 were taking air from both the working chamber 3 and from the vacuum store chamber 4, and if the control valve 8 were to remain open all the time, the suction in both the said chambers would follow the full-line curve 3a+4a near the middle of the graph, the suction increasing gradually towards P(stat) The course of the suction really obtained in the working chamber 3 is represented by the broken line 3a+4a=F(8) which starts at PlO) in the left lower corner of the graph, the control valve 8 being closed. When the pump 2 starts up the suction in the working chamber 3 rises rapidly, following the curve 3a until at the instant tut,), it attains the switching suction P)6).

This is the degree of vacuum at which the control valve 8 opens, and it is sufficient to operate the working chamber 3. After that, with the control valve 8 now open, the suction in the working chamber 3 remains constant at P)6) during the time interval delta t. During this period the suction in the vacuum store chamber 4 follows the line 4a until it also reaches the switching suction P(s,.

Finally the suction in the two chambers 4 and 3 follows the curve 3a+4a=f(8) up to the highest suction P(stat) which the pump 2 is capable of providing. In Figure 5 it will be observed that the period delta t is the time saved by control valve 8 in bringing the suction in the working chamber 3 up to what is necessary for a single operation of the vacuum-operated device of which it forms a part, compared with how long it would otherwise take to reach this necessary operational suction.

To summarise, the control valve 8 provides the working chamber with operational suction much more rapidly.

Figure 6 shows a control valve 8 suitable for use in an arrangement of the kind represented in Figure 3. The control valve 8 has an air outlet 10 leading to the pump 2 of Figure 3, an air inlet 11 leading to the working chamber 3 and an air inlet 17 leading to the vacuum store chamber 4. Again let it be assumed that before the pump 2 starts up the entire system is under atmospheric pressure.

Under these circumstances the diaphragm shown at 18 in Figure 6 is in the right-hand position shown. A valve plate 21 rests in contact with a valve seat 19, thrust by a spring 20. When the pump 2 shown in Figure 3, starts up it aspirates air through passages 10 and 11 from the working chamber 3, but it cannot take air from the vacuum store chamber 4 through passage 17 because the valve 21, 19 is closed. The working chamber is evacuated up to the switching suction P)6). At this instant the control valve switches open in that the diaphragm 1 8 moves leftwards, against the influence of a spring 24, driven by the pressure difference between the atmospheric chamber 23 at its right and the chamber 22 at its left, which receives suction from the pump 2.In this leftwards movement of the diaphragm 1 8 an actuator pin 25 of the diaphragm 18 thrusts a valve shaft 26 leftwards, lifting the valve plate 21 from its seat 19 against the influence of its spring 20 and opening the valve, allowing the pump 2 to evacuate the vacuum store chamber 4 through passage 17. The control valve 8 has dimensions so that the valve 21, 19 opens sufficiently to ensure that the flow resistance through it corresponds to the pressure difference across it.

The evacuation of the vacuum store chamber 4 continues up to the switching suction P)6). After that the suction in both 4 and 3, which now are equal, increases up to the highest suction the pump 2 is capable of delivering, the valve 21, 1 9 opening as far as its mechanical stop. The switching suction P(s) is determined by the strength of spring 20.

During the operation of the motr vehicie incorporating the arrangement, if the suction in the working chamber 3 falls to below the switching suction P(s) the diaphragm 18, moving towards the right, lifts the actuator pin 25 out of contact with the valve shaft 26, closing the valve 21, 19 and allowing the pump 2 to evacuate the working chamber 3 only. During this period, in which the working chamber only is being evacuated, the valve 21, 19 acts as a non-return valve, fulfilling the function of the non-return valve 6 in Figure 4. Thus the control valve 8 contains integrally not only the T-connection 10, 11, 16 and the passage 17 of Figure 3, but also the non-return valve 6.

Figure 7 shows the arrangement of Figure 4 containing a simple control valve 8 and connected to a vacuum servo braking system. Before actuation of the brake pedal 27, the valve 29 is closed and valve 30 is open. Consequently, with the pressure equal on its two faces, the diaphragm 28 is in its leftwards position thrust by its spring 31. Actuation of the brake pedal 27 not only thrusts the brake actuator rod 32 rightwards, but also closes valve 20 and opens valve 29, with the result that atmospheric pressure in chamber 33 thrusts the diaphragm 28 rightwards, the chamber 34 having suction. Diaphragm 18, moving rightwards, thrusts the hydraulic piston 35 rightwards and applies the brake with the help of hydraulic fluid 2, brake pistons 36, pads 37 and the brake disc 38.When the brake pedal 27 is released, valve 29 closes and valve 30 opens, equalising the pressures in chambers 33 and 34, allowing the spring 31 to return the diaphragm 18 to its leftwards position of rest.

A further embodiment of the control valve 8 shown in Figure 8 also contains, as described before, the T-connection 10, 11,16 shown in Figure 4, the passage 1 7 and the non-return valve 6. But it also contains, as a further development, a valve 42, 39 comprising a valve plate 42 and a valve seat 39, which fulfils the function of the non return valve 7 of Figure 4.This valve ensures that after the pumps 1 and 2 have been started up, and when the switching suction P(s, is reached, causing the control valve 8 to switch over so that pump 2 now evacuates the vacuum store chamber 4, the working chamber 3 is isolated from pump 2, allowing pump 1 to continue evacuating the working chamber 3, the discharge from pump 1 being evacuated by pump 2, i.e. the two pumps functioning in series with each other in the evacuation of the working chamber 3. The effect obtained is that after the switching suction P(s) has been reached the suction in the working chamber 3 continues increasing, rather than remaining constant during the interval delta t, as described by the broken line in Figure 5.When the two pumps 1 and 2 are started up, at the point P(o) in Figure 5, they first evacuate the working chamber 3 in parallel with each other. In Figure 8 the diaphragm 1 8 is at first in its upper position.

Actuator pin 25 is not in contact with valve shaft 26. Valve 19, 21 is consequently closed. The valve 42, 39, which mimics the non return valve 7, is held open by the suction of pump 2. On reaching the switching suction P(s) the actuator pin 25 descends, opening the valve 19, 21, which leads to vacuum store chamber 4. The resulting increase in pressure in chamber 41 closes valve 42, 39, isolating the working chamber 3 from the pump 2. After that, pump 1 continues evacuating the working chamber 3, while pump 2 evacuates both the vacuum store chamber 4 and the air discharged from pump 1. Figure 8 shows the spring 43 of the valve 42, 39 supported by the actuator pin 25, but it can, if desired be supported by the housing of the device. The valve plate 42 slides up and down between two stops 47 on the actuator pin 25.

Although the control valve 8 which has just been described is primarily for the 2 pump arrangement of Figure 4, it can, if desired be used in the 1 pump arrangement of Figure 3, although in this case the passage 9, with its non-return valve 6, must be retained. The continued evacuation of the working chamber after the switching suction P(s) has been reached, nevertheless takes place. Figure 9 shows a further development of the arrangement of Figure 8, but with the addition of a valve 45 to mimic the non return valve 5 of Figure 4. Before the switching suction P(s) is reached the pump 1 discharges through passages 14, 1 7, chamber 40, valve 45, against the influence of its spring 44, and chamber 46 to the atmosphere. The valves 45 and 21 function in parallel with each other.

Claims

1. A method for operating a motor vehicle vacuum system comprising a user provided with a working chamber for affecting a vacuum-operated action, and a suction pump for evacuating said working chamber; characterised in that in a first stage the said working chamber is evacuated by the said suction pump to a degree of vacuum which is sufficient for a single vacuum-operated action of the user, and in a further stage the said working chamber and a vacuum store chamber are evacuated simultaneously by the said suction pump to the maximum degree of vacuum capable of being effected by the said suction pump, and valve means for applying vacuum in the said vacuum store chamber to the said working chamber when the pressure in the said working chamber exceeds that in the said vacuum store chamber.

2. A method according to Claim 1, characterised in that the said first and further stages of evacuation are regulated by gas flow control means between the said working chamber and the said vacuum store chamber and connected also to the said suction pump, the said gas flow control means being disposed in parallel with a non-return valve constituting the said valve means which non-return valve is closed during the said first and second stages of evacuation and which is opened when the suction requirements of the said user exceeds the capacity of the said suction pump so that the pressure in the working chamber exceeds that in the vacuum store chamber, whereby the said working chamber is evacuated by the said vacuum store chamber.

3. A method according to Claim 2, characterised in that the said gas flow control means is a throttle means, whereby action of the said suction pump evacuates gas in the said first stage simultaneously from the said vacuum chamber and the said working chamber, the gas being evacuated from the said vacuum chamber flowing at a slower rate therefrom than from the said working chamber.

4. A method according to Claim 2, characterised in that the said gas flow control means is a control valve adapted to open at a predetermined degree of vacuum, whereby when the said control valve is closed, operation of the said suction pump evacuates the said working chamber in the said first stage, and the said working chamber and the said vacuum chamber are evacuated simultaneously in the said further stage with the said control valve open.

5. A method as claimed in Claim 1 for operating a motor vehicle vacuum system, substantially as hereinbefore described and illustrated in any of the accompanying drawings.

6. An arrangement for operating a vehicle vacuum system, comprising a user provided with a working chamber for effecting a vacuumoperated action, a suction pump for evacuating the said working chamber to effect a single vacuum-operated action of the user, a vacuum store chamber connected to the said pump whereby the said vacuum store chamber may be evacuated simultaneously with the said working chamber to the maximum degree of vacuum capable of being effected by the said pump, and valve means for applying the vacuum generated in the said vacuum store chamber when pressure in the said working chamber exceeds that in the said vacuum store chamber.

7. An arrangement according to Claim 6, comprising the said user provided with a working chamber (2-Fig. 1) and a vacuum store chamber (3) connected in parallel to the said suction pump (1), and a throttle (4) disposed between the said working chamber and vacuum store chamber to restrict the flow of gas therebetween compared with the rate of flow of gas from the said working chamber during operation of the said suction pump, the said throttle being disposed in parallel with a non-return valve adapted to be maintained in a closed position until the pressure in the said working chamber exceeds the pressure in the said vacuum store chamber.

8. An arrangement according to Claim 7, in which the said throttle and the said non-return valve are built into the said vacuum store chamber.

9. A modification of the arrangement of Claim 7 or Claim 8 wherein the said pump (1) is a pressure pump, and the said non-return valve is adapted to be maintained in a closed position until the pressure in the said working chamber becomes less than the pressure in the said vacuum store chamber.

10. An arrangement according to Claim 6, comprising the said user provided with a working chamber (3-Fig. 3) and a vacuum store chamber (4) connected in parallel to the said suction pump (2), and a first non-return valve (6) disposed between the said working chamber and vacuum store chamber adapted to be maintained in a closed position until the pressure in the said working chamber exceeds the pressure in the said vacuum store chamber, the said non-return valve being disposed in parallel with a control valve (8) between the said vacuum store chamber and the said pump and adapted to connect and disconnect the said suction pump and the said vacuum store chamber, and a further non-return valve (7) disposed between the said control valve and the said working chamber, the said further control valve being adapted to be maintained in a closed position when the said control valve is open.

11. An arrangement according to Claim 10, wherein there is disposed between the said working chamber and the said vacuum store chamber a further suction pump (1-Fig. 4) in parallel with the said first non-return valve adapted to discharge gases being evacuated from the said working chamber by operation of the said both suction pumps when the said control valve is closed.

12. A control valve for use in the arrangement according to Claim 10 or Claim 11, comprising a housing, a first connection (lO--Fig. 6) for a suction pump of the arrangement, a second connection (11) for the working chamber of the arrangement and a third connection (1 7) for the vacuum store of the arrangement, a valve (19, 21) interposed between the said second and third connections and having a diaphragm (18) which is acted on on one face by atmospheric pressure and on its other face by the combined effect of a biasing spring (24) and a vacuum generated by operation of the suction pump of the arrangement, the said diaphragm having an actuator pin (25) which opens the said valve (19, 21) by thrust of its valve shaft (26) inwards against the influence of a further valve spring (20).

13. A control valve according to Claim 12 comprising two stops (47-Fig. 8), a valve plate (42) located and slidable between the said stops against the influence of a spring (43) and a valve seat (39) forming with the said valve plate a second valve.

14. A control valve according to Claim 12 and Claim 13, which further comprises a non-return valve (45-Fig. 9) in parallel with the said valve interposed between the said second and third connections adapted to vent to the atmosphere.

1-5. An arrangement according to Claim 6, substantially as hereinbefore described and illustrated in any of Figures 1 to 4 of the accompanying drawings.

16. A control valve for use in the arrangement according to Claims 10 or 11, substantially as hereinbefore described and illustrated in any of Figures 6,8 and 9 of the accompanying drawings.

1 7. A servo-assisted motor vehicle vacuumoperated brake incorporating the arrangement according to any of Claims 6 to 11.