A residual valve was included in the M/C on all-drum systems way back when, to keep the brake lines full and prevent air from being pulled past the wheel cylinder cups when the pressure at the brake pedal went away. The brake shoe return springs were sufficient to overcome the residual pressure and allow the shoes to move away from the drums and prevent drag. Since disc brake caliper pistons have no return springs, little or no residual pressure can be allowed against the calipers. Disc pads are intended to have a slight drag on the rotors in most systems to act as a squeege after a disc is exposed to splashed water. It's also intended to prevent excessive pedal travel and to compensate for pad wear by allowing the pistons to follow the pads as they wear. Most caliper seals are square-cut, which provides a slight retraction of the pistons to prevent excessive drag as the pressure on the brake pedal is released.
Jerry
