The problem I have with residual valves is the part where the fluid drains back. Just how does it drain back? Is it pulling the disk caliper piston back into the housing? is it pulling the wheel cylinder pistons back? It can't be pulling air into the system, and fluid can't simply return leaving a vacuum in the line.

My is frame mounted, I have never had it overflow or had to pump the pedal.

Just how low of frame do you have if the master is lower than the center of the wheels, because all brake cylinders are above the center of the wheels. These trucks all had frame mounted master cylinders with out residual valves when new, so do most of the old cars, how did they work for all these years?