an accumulator assembly for a vehicle anti - lock brake system receives and temporarily stores fluid during pressure reduction phases of the anti - lock operation , and maintains fluid stored therein substantially at or near zero pressure during anti - lock operation . the accumulator enables the fluid pressure to the controlled brakes to be reduced to substantially near zero pressure . the accumulator is further operative , after braking of the vehicle , to apply a predetermined pressure above zero pressure to the brake fluid stored therein to enable the fluid to be returned to the brake system . the accumulator is advantageously used in a pumpless anti - lock system which is operative to control the rear disc brakes of a light pick - up truck .