If residual valves are used with disc brakes, they need to be VERY low pressure items. Calipers have no springs to retract the pads away from the rotors, so residual pressure in the lines above a pound or two will create drag and overheating. It also takes very little pressure to move the large diameter caliper pistons, compared to a drum brake wheel cylinder. The heavy spring pressure of drum brake shoe retracting springs allows much higher static pressure to remain in the lines, which also keeps the wheel cylinder cups expanded against the cylinder walls to avoid air leaks into the fluid.
Jerry
