Master cylinder with fast fill auxiliary chamber within a piston

In a fast-fill master cylinder, a housing forming a reservoir containing fluid and a bore movably receiving a pair of pistons, the bore including a wall forming a substantially uniform diameter extending over the entire length of the housing, the pair of pistons cooperating with each other and with the housing to substantially define a pair of pressure chambers which normally communicate with the reservoir in a rest position for the pair of pistons, one of the pair of pistons defining an internal auxiliary pressure chamber within the one piston normally communicating with the reservoir via a passage having a valve assembly therein, the auxiliary pressure chamber receiving a position sensitive member cooperating with the one piston to contract the auxiliary pressure chamber in response to movement of the one piston away from the rest position, and check valve means carried by the one piston permitting one way fluid communication from the passage to one of the pair of pressure chambers, the one piston being movable initially away from the rest position during a brake application to generate fluid pressure within the auxiliary pressure chamber, the check valve means providing less restriction to fluid flow than the valve assembly when the fluid pressure generated in the auxiliary pressure chamber is below a predetermined value so that the generated fluid pressure is communicated to the one pressure chamber via the check valve means when the generated fluid pressure level is below the predetermined value and communicated to the reservoir via the valve assembly when the generated fluid pressure level is above the predetermined value.