and master cylinder should also be inspected. If the vehicle comes through the door with brake application complaints like rear wheel lockup or premature disc brake application on slick road surfaces, it’s possible that the combination valve is inoperative. If you’re servicing an ’80s or earlier ­vehicle, be aware that most drum brake master cylinders contain a residual check valve that increases pedal response by maintaining a small amount of hydraulic pressure in the system. To test for residual pressure, firmly apply the brake pedal and then check for a small spurt of fluid when the wheel cylinder bleeder screw is opened. ­Although it’s old technology, be aware that installing a drum brake master cylinder equipped with residual check valves on a disc brake system will cause the disc brakes to drag and wear prematurely. Remember also that residual check valves were designed out of later drum brake ­designs because they were no longer required. 

                                                               DRUM BRAKE SERVICING TIPS