Where a master cylinder is concerned, "new" doesn't necessarily mean "good". The symptom you're describing is classic master cylinder piston seal failure. Pumping the pedal allows the cup seal to expand, but as soon as the pedal pressure is released the cup shrinks. If you adjust the linkage far enough forward to keep pressure on the brake lines, the cup stays expanded, but the brakes drag.

Keep changing the master cylinder until you get a good one.
Jerry

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