On vehicles equipped with front disc and rear drum brakes, a proportioning valve is an important part of the system. It is installed in the hydraulic line to the rear brakes. Its function is to maintain the correct proportion between line pressures to the front and rear brakes. No attempt at adjustment of this valve should be made, as adjustment is preset and tampering will result in uneven braking action.
To assure correct installation when replacing the valve, the outlet to the rear brakes is stamped with the letter R.