Compound master brake cylinder

A compound master brake cylinder assembly adapted for use in a hydraulic braking system, said system having braking elements connected by an outlet to said compound master brake cylinder assembly, said assembly comprising:

• a fluid reservoir, a first chamber defining structure adapted to be placed in communication with the reservoir, a first piston slidable relative to said first structure, a second chamber formed within the first piston and adapted to be placed in communication with the outlet, a second piston slidable relative to said second chamber between first and second positions, force applying means capable of initially causing the first piston to move relative to the first chamber defining structure, and to subsequently cause the second piston to move relative to the second chamber, and relief valve means extending between the first chamber and reservoir and responsive to outlet pressure and first chamber pressure and capable of venting fluid from the first chamber to reservoir while progressively reducing first chamber pressure in response to progressive increase in outlet pressure above a predetermined limit whereby the force applied by the force applying means does not substantially vary while the first chamber pressure is being reduced after the outlet pressure exceeds the predetermined limit, the relief valve means including a poppet valve normally spring biased to a closed position, first pilot operated means responsive to outlet pressure, and second pilot operated means responsive to first chamber pressure, said first and second pilot operated means acting against the poppet valve to vent fluid from the first chamber.