The Stovebolt Site Forums The Shop Area Driveline Residual valves, proportioning valve etc.

The old gm masters had internal Residual valves built in. Most Masters that have a drum configuration have them in. Here are the imagesI have seen in the past on 50s manuals. In theory you should not have air in system via drain back. Think finger on straw. But My guess was engineers wanted to eliminate it from happening. If something started leaking downstream and the larger springs on drum brakes pushed back.