one other thing to remember about drum brakes. it usually takes about 50-75# of pressure to overcome the return springs, so the brake shoes always retract to their fullest position unless there are other problems present, such as "wear slots" in the backing plates, or broken or incorrectly installed springs.
cab, sorry if i gave the impression to others that there was no use for RPVs. that was not my intent. total system analysis should be preformed to decide if you need to install these aftermarket valves or not.