I've read that the master cylinder doesn't allow the return of fluid from the brake system until it's completely retracted. If it's out of adjustment, the plunger won't let fluid back out of the system, locking the brakes. Never happened to me, but I've heard of it.
Good luck to you, I'm doing a full brake replacement on my '51 and may run into the same problem. Seems to me there is a section on adjusting the pedal free play in the shop manual.
David Bush
