GB2024353A - Mixed braking system - Google Patents

Abstract

A control system for pressurised medium-operated friction brakes combined with a hydrodynamic retarder of a motor vehicle comprises a first pressurised medium source 1 connected by ducts 7;7c to a hydrodynamic retarder 2 which has a pressure medium actuated control member 11. A second and a third pressurised medium sources 3 and 5 supply the medium via respective ducts and control valves 12a; 14; 12b and 13a; 15; 13b to friction brake assemblies 4 and 6 of the vehicle. The duct 7 divides into two branches 7a and 7b which respectively contain control valves 9a and 8. The valve 9a is operated by a pedal 16 and controls the flow of medium to the retarder 2 during short-term braking. The manual valve 8 controls the flow of medium to the retarder 2 during long-term braking. The two branch ducts 7a and 7b re-unite in an automatic shuttle valve 10 which has two alternatively blockable inlets. The shuttle valve 10 is so constructed that the inlet carrying the medium at higher pressure is connected to the outlet 7c whilst the other inlet carrying the medium at lower pressure is blocked against return flow. The pedal 16 is arranged to operate successively the valve 9a, for controlling the retarder 2, and the valves 14; 15 for controlling the friction brakes assemblies 4;6. <IMAGE>

Description

SPECIFICATION A control system for pressurised-medium-operated friction brakes combined with a fluid retarder of a vehicle having a change-speed gearbox This invention relates to a pressurised-mediumcontrolled adjusting member system for service brakes combined with retarders of vehicles with change-speed gearboxes.

It is known for the purpose of controlling brakes in vehicles to use circuits with pressurised-mediumcontrolled or electric adjusting member systems which permit an operation of the motor vehicle friction brakes and of a wear-free brake acting in the driving axle, e.g., a fluid brake (also called a retarder) through a common actuating member. It is also known to use in this case so-called two-circuit treadle-operated brake valves which have suitable equipment to coordinate the various brakes with one another in such a way that, e.g., in the starting range of any desired braking operation only the wear-free retarder brake, and at the end only the friction brake comes into function.Also known from this is the problem arising for the designer concerned both for maximum safety and also for optimum comfort, who under the constraint to avoid sources of faults (such as e.g., spring fatigue, increased friction due to wear and consequences of heating) must ultimately also devise solutions which will prove to be comfortable and sure in operation from the user's standpoint and also inexpensive and versatile in usefulness for the manufacturer, under the strongly fluctuating conditions of service.

The obstacles confronting the designer in working towards polyvalent solutions are attributable more particularly to the differences in construction and to the resulting different modes of behaviour of the various retarder types and installation methods.

Whereas where the retarder is placed in front of the change-speed gearbox- i.e., where it is a "primary retarder", an adequate braking action is obtained down to low speeds (e.g., approximately 15km/h), nevertheless on the other hand the disagreeable accompanying phenomenon of speed change shocks when changing speed downwards renders special countermeasures necessary, on the other hand where the retarder is placed behind the change-speed gearbox, i.e., where it is a "secondary retarder", the braking action of the retarder is practi callyfinished even at much higher speeds, however no speed change shock is to be feared in downward speed changes. Different conditions again apply to so-called integrated retarders - i.e., where the torque converter is used as a brake by flow reversal.

In the course of attempts to find as ideal a solution as possible to all the various desiderata resulting from these differences a brake control principle substantially characterised by electrical changeover contacts and solenoid valves for various control actions has been introduced on a wider basis (see i.e., House Journal "atü-Brake technique" of Messrs WABCO-Westinghouse GmbH, Hannover, Edition 1/1977).

Forthe practical case of a so-called integrated primary retarder- where the converter is used as a brake by flow reversal - which would no longer operate sufficiently reliably due to its installation position and mode of construction and also due to the high speed dependence, a changeover apparatus has been provided in the system here for example which changes over automatically to the normal friction service brake as soon as the speed has fallen below this lower limit.

Unfortunately however this solution is still unsatisfactory in many respects. A large number of electrical switch contacts and the solenoid valves controlled from them cause both jerky transition from braking range to braking range and also increased risk of error in installation, maintenance and use. Thus it was stated even in German Published Specification 2 231 090 that it is disadvantageous when an adjusting member system for vehicle brakes relies on a large number of electric switches requiring to be adjusted differently and more prone to fault and hypersensitive compared to pressure control valves or when it is based to a large extent upon solenoid valves, the abrupt operation of which makes soft transitions from one braking range to another difficult.

Attention has already been drawn expresslytothe familiarity problems resulting from hitherto customary brake actuating systems and clearly prejudicial to safety for drivers who frequently have to control alternately different types of omnibuses or heavy lorries, with different loads, dimensions and braking functions, in the report "Retarders for Commercial Vehicles" (INST. MECH. ENGRS., Conference Publication 1/1974).

It is obvious that such added difficulties and risks are caused not only by differently adjusted braking systems but also by the hitherto customary large number of braking range steps and adjusting devices and the effects of the changeover components necessary for them. More particularly, the lack of a clearly detectable separation of retarder braking range and service braking range has made it difficult to assess the braking force reserves and sensitive dosing.Thus with the solutions hitherto known it was e.g., not a simple matter even for experienced users, without long practice, to distinguish reliably and rapidly between the various pressure points on the pedal travel which have to signal the commencement of the generally more than two retarder braking steps and e.g., to evaluate them fast enough and accurately enough e.g., for the purpose of estimating the braking force reserves still available. These difficulties were further aggravated by the typical construction influences of the many different types of retarder construction. Thus e.g., control systems became known which initiated a switching off of the retarder after the last of not more than four braking steps, which had generally been made detectable by pressure points on the brake pedal.

Other systems, more particularly with electric retarder, do not switch it off even after the last step.

Now although this reduces the risk of overbraking, nevertheless in both cases the driver is unable to detect reliably, more particularly due to the many braking steps, how much braking force reserve is still available or from what position of the brake pedal the wear-free braking stops. Even those con trol systems which exhibit no pressure points on the pedal travel at all and wherein practically no counterforce is detectable on the pedal travel, are found disadvantageous by many users because in this case a "stumble" into the forced braking range with possibly dangerous overbraking can occur due to slight inattention by the driver or e.g., by slight road shocks.

A definite structural and functional separation of the two brake systems, which are so disparate in their essence, is also to be called for from technical inspection considerations, more particularly in order to be able to ensure that when one system is repaired the next one may be interfered with, which might possibly result in the failure of both. The lack of a clear separation of the two systems in the constructions which have hitherto become known also had the result that it was impossible - or only possible for a considerably great outlay -to use the fluid brake selectively, in parallel with the motor vehicle friction brake and retarder system, in a finely dosed manner through a pressurised-medium-supplied manual lever regulator valve without having to perform a separate changeover.

Many known brake systems also have the following further functional disadvantages: - Depending upon the speed of actuation of the pedal valve, shock effects of different strength are obtained under otherwise equal conditions.

- Their strength is again highly dependent upon the volume of the following lines, even for an otherwise equal head pressure.

- The pressure variations in relation to the pedal angle differ.

- The forces exerted on the pedal in relation to the braking intensity or to the pedal angle, are greatly different.

- The force jumps at pressure points are often not approximately identical and difficult to distinguish, particularly in haste, due to the large number of them.

From this incomplete account of the problems due to the many different modes of control, the necessity for both a systematic analysis of users' requirements and a consequentfundamentally novel mode of control for braking systems of this region becomes clear. The result of such an analysis of requirements from the principal standpoint of the applicability to braking systems which are combined with fluid brakes can now be summarised in its essential points in the following list of desiderata, the greatest possible satisfaction of which is simultaneously the aim of the invention stated in claims 1 and 2 and also in the following subordinate claims.This invention is intended as a further development of features of pressurised-medium-actuated adjusting member systems of service brakes combined with retarders in vehicles in concordance with the relevant standa rds and regulations in order that a) instead of multiple-step controls, those with no step or with at most one step are permitted; b) instead of electric switch members, pressurised-medium-actuated members are used as far as possible; c) instead ofthefeeding in of energy from only one source, feeding occurs from separately supplied networks; d) instead of a plurality of brake control systems specific to the gearbox, only one adaptably flexible standard pedal principle is necessary;; e) instead of an integrated or sandwich-like mode of acommodation into the braking system, all the auxiliary components remain separate and accessible without interfering with the principal brake system; f) instead of an uncontrollable pedal travel/braking force ratio a steady pedal force increase can be obtained with only a very slight force increase at the end of the retarder regulation travel; g) instead of a complicated association of two different types of brake systems a simple possibility for a mode of actuation capable of parallel functioning and conveniently mutually augmenting is created; h) instead of a restriction to pedal actuation the same system in principle can, even without switching over, be brought into function for long-term brakings selectively also through a finely dosable manual lever regulator valve;; i) instead of an increased physical and sensory burden for improved brake equipments, only a com parativelytrivial additional burden need be imposed upon drivers and service engineers.

The pressurised-medium-controlled adjusting member system for service brakes combined with retarders of vehicles stated according to the patent claims satisfies all these desiderata in a virtually complete manner and therefore has a large number of advantages compared to the prior arL More particularly, the invention achieves the advantage for the user that the use of a retarder braking system now necessitates virtually no change of habits for drivers dealing with retarder brakes for the first time, because a stepped pressurised medium actuation corresponding optimally to the pedal force exerted and attacking much more gently than with solenoid valves exists by means of footplate for all the braking force ranges and it has a favourable construction from the standpoints of maintenance and inspection, which also permits without any switching over the arbitrary use of a selectively conjointly usable manual lever valve.

Furthermore the progress in safety technique which can be achieved by the various features of the invention is of great importance. It is now possible in case of need, even after the failure of the electrical system of a vehicle - still to brake with retarder and/or friction service brake even if one of the pressure medium sources or pressure lines should be interrupted, without any switching over manipulation whatsoever being necessary for this purpose, for which no more time might be available in an emergency.

Lastly, with the adjusting member system according to the invention operating and travelling comfort are also improved and the wear on the friction brakes is reduced because - even in the case of the primary retarder braking systems which are more sensitive to speed jumps - shock-like transitions in the course of deceleration and accidental strong braking are avoided in the most favourable manner.

According to claim 2, a possibility has been found as a further development of the invention, also an arrangement for the pedal operating element (9) which can be produced at favourable cost and is advantageous for maintenance and repair, which also more particularly facilitates the adaptation of this principle to the different desiderata dictated by the type of vehicle.

According to claim 3 an advantageous chronological sequence of the movement cycles of the various partial elements of the pedal operating element (9) has been found which makes possible more particularly the most favourable sensory detection of the various brake loading steps.

According to claim 4 features have been discovered as a further development of the invention how the achievement of the aim can be realised with a pedal operating element if a pivotable pedal is used for it.

According to claim 5 a mode of force distribution of the partial elements of the treadle operating elements (9) which is particularly advantageous for achieving the aim according to the invention has been discovered for a stepless actuation.

According to claim 6 a solution for actuation with a single definitely detectable and clearly associable force jump immediately on engagement of the friction service brake has been discovered.

According to claim 7 further possibilities are disclosed for solving problems intrinsically alien to the object with the adjustment arrangement principle according to the invention, in which additional electric signal and adjusting members cannot be omitted.

In order that the invention may be more readily understood, reference is made to the accompanying drawings which illustrate diagrammatically and by way of example an embodiment thereof, and in which: Fig. 1 shows a circuit diagram of the system according to the invention, Fig. 2 shows a treadle operating element, and Fig. 3 shows a brake pressure graph.

The construction of the system is explained with reference to Fig. 1, where a pressure medium source 1 for a retarder brake 2 and a pressure medium source 3 for a first friction service braking circuit 4 and a pressure medium source 5 for a second friction service braking circuit 6 are coupled together circuit-wise by means of the adjusting member system according to the invention in such a way that an energy failure in one of the systems cannot interrupt any other one.Whilst a retarder brake pressure line 7 divides up in front of a manual lever valve 8 or a modulator pressure regulator valve 9 into branch lines 7a and 7b which are recombined in a two-way valve 10 after passing the said regulating valves into a control pressure line 7c and ending in a charging pressure regulator valve 11 of the retarder 2, each of the two friction service brake pressure lines 12a and 12b or 13a and 13b is passed unbranched via its respective single pressure regulating valve 14, 15 to the associated brake 4, 6.

For monitoring purposes, or in order to connect the adjusting arrangement system according to the invention to other adjusting arrangements which may possibly also be present, it is of course possible to incorporate further signal and control elements into the control pressure lines 7, 12, 13 or to the operating elements 8,9 without modifying the essential features and advantages of the circuit according to the invention. However, since such auxiliary devices are not of decisive importance for an understanding of the system according to the invention they have not been included in the drawing or in the description here.

In Fig. 2 a treadle or a pedal 16 secured in a pivot bearing 17 with a main return spring 18 and with an auxiliary modulating spring 19 in the floor plate 20 of a vehicle so that with an overall pedal angle a of preferably approximately 21 to 25 , the adjustment of all relevant strokes is possible for the pressure regulator valves 9, 14, 15 likewise let into the floor plate 20.

The present embodiment shows an arrangement in which a tappet spring 22a of the modulator pressure regulator valve 9a beneath the shorter lever arm of the pedal 16 is so far compressed in the initial position due to the return forces of the springs 18 and 19 that the control piston in the modulator pressure regulator valve 9a occupies the venting position over the outlet 96.

For an actuation up to angle or approximately a = 7" with this arrangement, only the modulator pressure regulator valve 9a is engaged. Only at angles of a 70 does a delivery of pressure occur from the friction brake pressure regulator valves 14 and 15 which are juxtaposed in this embodiment, and of which only one is visible in the figure (due to the side elevation). However, the tappet 21 comes into abutment against the pedal 16 approximately 1" earlier.

The stop limitation adjustment possibility is provided here in the form of a setscrew 23 let into the floor plate 20.

The function of the system according to the invention will be further explained schematically with reference to a graph according to Fig. 3.

Here a denotes the angle pivoted by the pedal 16 out of the initial position (Fig. 2), PM the control pressure for the retarder brake actuation caused by the modulator pressure regulator valve Se, PB the service brake pressure caused by one of the two friction brake pressure regulator valves 14, 15 and PKthe force to be exerted upon the centre of the pedal 16; A p is designated as the region of the regulation hysteresis of the modulator pressure regulator valve 9. A PK is the force jump upon use of the service brake, which may also be omitted.

For PK, approximately 1 to 1.5 kp are necessary in order to set the pedal 16 in motion out of the initial position (a = 0 ). In order to ensure complete switching off, a fu nctionless starting stroke of approximately 1/20 of the overall stroke is provided here initially. After this the function of the modulator pressure regulator valve 9a commences with initially only a relatively slight steadily rising force requirement, namely from a = 0 with approx. 1 kp to approx. 5 kp at a! = 1/3. In this range (from point "A") the retarder control pressure pm rises from 0 to max., although a certain scatter is unavoidable depending upon the spring quality and similar influences.In order to ensure good regulabilityfor small braking torques, not more than approx., 1/16 of the total braking pressure should be obtained at 1/20. Moreover the pressure regulation behaviour of the modulator valve 9a leads to a regulation hysteresis A PM between P max. and P min., due to variations in the secondary pressure PM. If the control pressure drops bymorethan approx., 1/16 atthe maximum 1/3 or more travel, then the modulator valve 9a must be boosted.Over the entire distance from 0 or 1/20 to 1/3 of the total travel the friction brake pressure regulator valves 14, 15 are completely closed, i.e., for a linear pressure rise of PM braking occurs only by the retarder, however this already an idle stroke for the service brake 14, 15.

As advance warning of the engagement of the friction service brake 14, 15, a change of direction of the force increase free from force jump may be provided just before the end of the first third (at "B"), whereupon - after overcoming the single force jump of the order of magnitude of approx. 0.5 to 110 kp after 1/3 of the total travel at "C" for the sensory signalling of the engagement of the friction service brake 14, 15 it continues with approximately the same slope up to the end of the actuation travel at D.

The braking pressure Pe of the friction service brake is increased from 0 to max., with approximately the same slope as the retarder control pressure Pm and attains approximately at 2/3 of the total travel (at "D") its maximum value - i.e., the point from where the joint braking force Pm + Pecan no longer be increased even with more pedal force PK.

If, as shown in Fig. 2, an actuating element which is constructed as a pivotable pedal is used for the braking pressure regulator valves, then the following guiding principles for the selection of the spring characteristic have been found particularly advantageous for the system according to the invention: - in the initial range approx. 0.5 bar per pedal angle until just before the end of the first third of the travel (at point "B"), - thereafter approx. 0.56 bar per pedal angle to the end. The starting function from 0 to approx. 1/20 has been neglected here for the sake of simplicity.

Analogouslythe same conditions would be preferred where it is intended to use a pedal element with linear stroke characteristic. It is of course also possible for a construction corresponding in its effect to the solutions of the aims according to the invention to be achieved with differently arranged or dimensioned or more or fewer springs or other lever and lever length ratios and similar conversion measures, in which case the essential characterising features of the invention should still be retained.

Claims (10)

1. Pressurised-medium-controlled adjusting member system for service brakes combined with retarders of vehicles, comprising at least one pressurized medium source, a friction service brake system of any desired type, a hydrodynamic fluid brake or a converter useful as such in the driving axle of the vehicle with changespeed gearbox, a pressurised-medium-actuated regulating member of the retarder brake, preferably as charging regulator, at least one pedal-actuated operating element for the control pressure of the retarder regulating member, principally for short-term brakings, selectively a further optionally manually actuated operating ele ment for the control pressure of the retarder-brake regulating element, principally for long-term brak ings and the necessary pressurised-medium-stored control lines for the actuation of said elements, characterised in that mutually independent pressurised medium sources are present for the control of the friction service brake and for the control of the retarder brake, the principal pressure line from a pressure medium source supplying the retarderbrake system, which is independent of the pressure medium source/s supplying the friction brakes divides in the direction of flow into two partial sections which are preferably dimensioned for equally intense and equally rapid discharge of the maximum modulable flow in each case, namely either controllable exclusively through the pedal-actuated selfreadjusting operating element for preferably shortterm brakings or dosable exclusively through the preferably manual-lever-actuated operating element remaining in its adjusted position for long-term brakings, and also simultaneously or consecutively subdividable for any desired partial quantities of the total flow by means of both operating elements, and that there is further located in the control line branches departing from said operating elements an exclusively automatic change valve of type known perse reuniting them, which has two blockable inlets and one outlet and which is constructed so that the inlet with the higher pressure is connected to the outlet whilst the other inlet is blocked against reverse flow, and that exclusively the pedal-actuated operating element for the control of all the brakes is mechanically coupled to the pressure regulating valve/s for the control of the friction service brake and to the pressure regulator valve for the control of the retarder brake.

2. Pressurised-medium-controlled adjusting member system according to the introductory definition of claim 1 and wherein the pedal-controlled pressure regulator valve is arranged in known manner, for preferably short-term brakings, for the conjoint simultaneous control of two pressure regulator valves guided mutually independently but accommodated in the same operating element for the separate friction service brake actuating member actuations, and wherein readjusting forces counteracting the actuating force over virtually the entire pedal travel and preferably initiated by means of springs and a preferably adjustable limit stop for signalling the full braking position are associated with the pedal guide or with the service brake valve actuating tappet associated therewith, and wherein both valves each have a venting outlet which are opened when the pedal occupies the initial position or in the no-load range, characterised in that in the same pedal operated element and arranged for the conjoint simultaneous actuation together with the pressure regulator valves there is fixed independently yet a third modulator pressure regulator valve serving exclusively for the retarder control, which has suitable devices to be able to act as a constant pressure regulator at least in the fully opened state, with an opening spring which is maintained stressed in the initial position of the pedal and with a control piston which has closed the retarder control line and opened the vent in the initial position.

3. Pressurised-medium-controlled adjusting member system as claimed in claim 1 or 2, wherein the service brake pressure regulator valves remain closed from approximately a first third of the total possible actuating travel of the pedal, however the modulator pressure regulator valve to the retarder control is partially opened from approximately one twentieth of the travel and it is fully opened and remains opened after approximately the same actuating travel of the pedal after which the opening stroke of the valves to the friction service brake control has commenced, and wherein from approximately the second third of the total possible actuating travel of the pedal said pressure regulator valves are likewise fully opened and remain opened conjointly with the modulator pressure regulator valve.

4. Pressurised-medium-controlled adjusting member system as claimed in any of claims 1 to 3, wherein the treadle is a pedal for the angular movement, to which not only the friction service brake valves and the readjusting spring associated with them to re-establish the initial position with closed passage and open vent, but also a further separate preferably articulately mounted spring element is articulated beneath the longer lever arm departing from the footplate pivot joint for the generation and variability of a further pedal readjusting force, and wherein the modulator pressure regulator valve for the retarder brake control is articulated, fixed totally independently of the other valves, beneath the shorter lever arm departing from the footplate pivot joint.

5. Pressurised-medium-controlled adjusting member system as claimed in any of claims 1 to 4, wherein the readjusting forces are utilized for the direct sensory detection of the braking force reserves still available at any time by the operator by means of a sensible increase in resistance before the entry into the friction service brake range (C to D) and wherein for this purpose the pedal force (PK) for commencing the footplate actuation amounts to approximately one twentieth of the maximum force necessary to adjust the maximum opening crosssections in all the conjointly actuated pressure regulator valves, and wherein the pedal force (PK) after a constant, but initially with slightly weaker and only just before the end, approximately of the first third, of the total possible actuating travel (a) by an increase occurring with slightly intensified rise attains a point (at B) which corresponds to the full utilization of the retarder braking power or the maximum control pressure at the retarder (PmaX) and which represents approximately one fourth to one fifth of the maximum force for PK;; and wherein at that point (at B in this case) the transition from the slight to the more intense pedal force increase occurs without pressure jump and with a scarcely detectable transition, approximately one twentieth of the possible total travel before the commence mentoftheopening movement (at "C") of the friction service brake valve, and wherein after this scarcely detectable transition to increased resistance the pedal force increase is virtually unchanged, further increasing in linear and stepless manner, up to the limit point of the total actuating travel of the footplate fixed by a stop.

6. Pressurised-medium-controlled adjusting member system as claimed in claim 4, wherein simultaneously with the attainment of the opening point of the valves to the friction service brake control (at "C") a definitely detectable additional resistance (APK) of preferably approximately one half to one whole kilopond is opposed to the pedal force, and wherein after this sole force jump the pedal force (PK) continues to increase linearly and steplessly with virtually the same slope as before the force jump over the entire actuating travel up to the limit point fixed by the stop.

7. Pressurized-medium-controlled adjusting member system as claimed in claims 1 to 6, wherein one or more changeover valves of any desired type are provided in the control pressure line to the retarder charge regulating member for further coupling and signal functions, which are connected through auxiliary control lines to the manual lever valve or to a brake pressure control switch.

8. A control system for pressurised mediumoperated friction brakes combined with a hydrodynamic fluid retarder of a motor vehicle, comprising a first pressurised medium source connected by first duct means to a hydrodynamic fluid retarder having a pressure medium actuated control member, and at least one further pressurised medium source connected by a second duct means having a control valve to at least one friction brake assembly, said first duct means dividing downstream of said first pressurised medium source into two branches respectively containing a first control valve, for controlling the flow of said medium to the retarder preferably during short-term braking, and a further control valve operable independently from said first valve for controlling the flow of pressurised medium to the retarder preferably during long-term braking, the two branches being re-united in an automatic change valve which has two blockable inlets and one outlet to the retarder and which is so constructed that the inlet carrying the medium at higher pressure is connected to the outlet to the retarder whilst the other inlet carrying the medium at lower pressure is blocked against return flow, a foot actuatable operating member being provided for successively operating said first control valve and the valve in said second duct means.

9. Pressurised-medium-controlled adjusting member system for service brakes combined with retarders of vehicles, substantially as herein described with reference to and as shown in the accompanying drawings.

10. Pressurised medium-controlled service brake installation for vehicle drives equipped with retarders, essentially comprising at least a pressurised medium source (3) pressurised medium dis tributor means for friction service brakes (12b, 4), a hydrodynamic fluid brake (2), a pressurised medium-actuated control member (11) of the retarder brake, at least a first pedal-actuated control element (9) for the control pressure of the retarderbrake control member (11), principally for short-term braking, a second control element 9'0 for the control pressure of the retarder-brake control member (11), principallyforlong-term braking and preferably hand-operated, and pressurised medium fed control lines for actuating these element, characterised in that the installation includes pressurised medium sources (1 and 3 and/or 5), which are independent from each other, for the control of the friction service brake (4 or 6) and the retarder-brake, the main pressure line p) from a pressure medium source (1) supplying the retarder-brake system divides in the direction of flow into two partial flows (7a, 7b) which are re-united by means of a change valve (10) and which are either infinitely-variably controlled exclusively via the first automatically re-settable control element (9) or infinitely-variably dosable exclusively via the second control element (8) which remains in its adjusted position and also simultaneously or consecutively automatically subdividable for any desired partial quantities of the total flow by means of both control elements (8, 9), and that the change valve (10) has two inlets (7a, 7b), blockablefrom outside by the driver of the vehicle, and one outlet (7c) to the retarder-brake control member (11), and that the change valve (10) is so constructed that the inlet carrying the medium at higher pressure is connected to the outlet whilst the other inlet is blocked against reverse flow, and that exclusively the first control element (9) is for the control of all the brakes, mechanically coupled, via tappets (21 and 22) controlled in dependence on actuating angle a, to the or each pressure control valves (14, 15) for the infinitely variable control of the friction service brakes (4, 6) and to the infinitely variably actuatable pressure control valve (9a) for the control of the retarder brake (2), which control valve (9a) is actuatable sol ely via thetappet (21) depending on the angle a.