My understanding is that residual valves haven't been needed since wheel cylinder springs changed to a smooth cup (or tight spring coil) that better supports the rubber cups, probably in the 1970's, but research for yourself. The purpose for both is to keep the rubber tight in the bore so no air gets past when the pistons retract. If you ever change to disk brakes, you must insure the MC has no residual valve in the disk port or they can cause the pads to drag.