I've always heard that residual pressure valves will cause disc brakes to drag, because there's no spring pressure to retract the pads, like a drum brake has. At least, when one of my friends used a master cylinder with anti-bleedback valves on his race car, all 4 disc brakes had enough drag to turn the rotors red hot. When we changed to a MC without the residual-pressure valves, the drag stopped happening.

It sounds like you need a MC with a bigger bore size, to displace more fluid on the first stroke.
Jerry

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