Hy The Hill Boys, if the master cylinder used was designed for a disc/disc, or disc/drum system but actually being used on a drum/drum system, then the circuits that were intended for disc brakes would need a 10 p.s.i. residual pressure valve installed. Residual pressure valves maintain a pressure in the lines of the drum brakes which helps to keep the piston cups against the wheel cylinder walls and that pressure also helps counteract the action of the brake shoe return springs, without the residual pressure valves the brake shoe return spring will force the wheel cylinder pistons together in the wheel cylinder, this action will cause the brake pedal to feel low on the initial brake application and then feel correct on subsequent applications. This action will occur every time the vehicle is allowed to sit unused for a time, the other issue that occurs without residual valves is air can be drawn past the wheel cylinder cups which can contaminate an alcohol based brake fluid with moisture causing rust to occur in the system and a possible spongy feeling brake pedal, hope that helps.
