Another alternative is that the pushrod on the master cylinder is adjusted a little too long. If it doesn't let the master cylinder piston come all the way back, it can't release the pressure back into the reservoir.

As an alternative to that alternative (i.e. nobody has messed with the brakes for a long time), the piston can just be a little sticky and not returning all the way because of that. You can pop off the dust seal and look inside with a flashlight to see if the piston is returning all the way to the circlip that stops it from falling out the back end.
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'63 PV544 rat rod, '93 Classic #1141 245 (now w/16V turbo)