the seal 306 on the second piston means 236 the mode of operation of the master cylinder assembly 210 is as follows an input force applied to the brake pedal is transmitted through push rod 114 to move piston 254 return spring 242 and separation spring 250 are of the same size and therefore are simultaneously compressed to allow face 274 on the first valve seat means 268 and seal 306 on the second piston means 236 to engage seal 282 and face 317 on the second valve seat means 308 at the same time to interrupt communication between the first and second pressurizing chambers 238 and 240 and the first and second passages 284 and 288 connected to the reservoir further input from the operator moves pistons 254 and 236 to develop first and second hydraulic fluid pressures for operating the first and second sets of wheel brakes 70 and 72 when the input force on push rod 114 terminates return springs 242 and separation spring 250 move the first and second pistons 254 and 236 against stops 248 and 256 to reestablish communication of fluid between the reservoir sections 226 and 228 and the first and second pressurizing chambers 238 and 240 through compensator passages 284 and 288 respectively the pressurizing piston means for the master cylinder assembly 410 embodiment shown.