Also, I had the same problem with my 69. Had to kinda "pump it" to get her to stop. So I installed a new master. Well, after getting that installed, I was pumping the brakes in the driveway (thank God), and On the 3rd pump, the pedal went striaght to the floor.
The pressure that the old master couldn't do and the new one could, blew the rear brake hose!!!
Long story short, new rear wheel cylinders, steel lines going to them, new rubber hose, and new steel line from the hose to the front of the cab.
Replace anything that does not look good. It's a PITA, but it worth it for your safty, and the safty of others.