A two-circuit brake system for use with motor vehicles having at least two axles, said brake system comprising:
- (a) a housing;
- (b) a brake valve device in said housing;
- (c) manually operable means for actuating said brake valve device;
- (d) first piston valve means connected to said manually operable means and operable, upon movement of the manually operable means up to a certain amount, for controlling the amount of pressure in a first pressure chamber in the brake valve device;
- (e) a relay piston assemblage operable to a braking position in response to pressure in said first pressure chamber and in a control pressure chamber acting jointly on a pressure surface area of the relay piston assemblage for controlling the amount of pressure in a second pressure chamber, said relay piston assemblage having a pair of intermediate control chambers formed therein adjacent said pressure surface area and connected, respectively, to the pressures in said first mentioned control chamber and in said first pressure chamber, said intermediate control chambers being separated from one another in a pressure-tight relationship,
- (f) said first pressure chamber being connected, via a vehicle load-responsive brake pressure regulator, with brake cylinders on one of said axles for effecting braking thereof according to vehicle load, and said second pressure chamber being connected with wheel brake cylinders on the other of said axles for effecting braking thereof according to the amount of pressure in the second pressure chamber, the braking ratio between said one axle and said other axle being in accordance with the ratio of the pressure in said first and control chambers relative to the pressure in said second chamber;
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- (g) a supplementary ring fixed concentrically in said housing and on the relay piston assemblage and which, along with an intermediate ring disposed within and concentrically relative thereto, acts to separate said intermediate control chambers in said pressure-tight relationship and to partly define the amount of said pressure surface area, being exposed to each of said intermediate control chambers said supplementary and intermediate rings being replaceable with respective rings of different dimensions so as to change the amount of said pressure surface area being exposed to each of said intermediate control chambers.
- (h) said relay piston assemblage being tensionally connectable through a stem member with the manually operable means and being operable, in the event of failure of fluid pressure in said first chamber and upon movement of said manually operable means in excess of said certain amount, to its said braking position.