+1 on the residual valve suggestion. If the master cylinder does not match the caliper, it's probably holding pressure on the piston. Drum brake M/C's have a check valve that holds a few pounds of pressure against the shoe return springs. Since disc brake calipers have no return springs, the residual pressure valves aren't required, and they can cause major brake drag problems if they're used. Disc brake M/C's also have a larger fluid reservoir for the calipers than for the rear wheel cylinders.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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