Question about installing master cylinder on 68 dart

Thanks. Unfortunately my old master cylinder did not have the keeper. I don’t know what prevented the piston from coming out. Maybe just the pressure of the throttlThanks. Unfortunately my old master cylinder did not have the keeper. I don’t know what prevented the piston from coming out. Maybe just the pressure of the brake pedal rod? Anyway I left that retainer on and have installed the new master and the brakes work fine