Residual pressure valves hold the minimum pressure required on either the front or rear brakes by preventing flow back to the master cylinder. They are usually used where the master cylinder is located below the level of the wheel cylinders to prevent this flow back to the master cylinder in order to provide full brake fluid to the wheel cylinders/calipers. All drum brake (4 wheel)master cylinders usually have these built in. Stock disc/drum setups may use a combination valve to provide the residual pressure and proportioning.