1937 Plymouth brakes

Working on a 37 Plymouth coupe. The brakes worked fine but when driving it the pedal would get higher and higher until the brakes would just lock up. After some research and reading I found a post here that said the brake pedal needed adjustment to let the piston in the master cylinder come back past the relief port. I did that and got the 1/4 inch free play before the rod contacts the piston. My problem is that now the pedal goes to the floor. I have bled to brakes and am positive there is no air in the system. There are no leaks in the system. I have also adjusted the brakes with the cam bolts until the wheel locks up and then backed them off just until the wheel frees up. Now, short of replacing the master cylinder I am at a loss. Any ideas? Thanks

Just verified... If I adjust the pedal (tightening it up) I get good pedal, but not until after the piston blocks the relief port. Then I am back to the same problem of the brakes locking up after driving a while.

Did you rebuild the wheel cylinders. If not, the pistons are aluminum and get corroded and can get stuck in the out position. That will lock the brakes up.

Wheel cylinders are all new. When the wheels lock up I can crack a bleeder and it frees them right up. This is due to the piston in the master cylinder being too far forward and covering the relief port. But when I adjust the pedal to let the piston move all the way back (and open up the relief port) I have no brakes.

Interesting to say the least. You apparently know what you're doing by your posts. It is the master cylinder not refilling it's reservoir and 'neutralizing' the brakes after the pedal comes up to home. (I can't see the wheel cylinders doing it as all would have to have the same fault at the same time. Highly unlikely.) I think you're going to have to bite the bullet and take the master cylinder off the car and put it on the bench. That's what I would do at this point because I'm just as mystified as you. If you have a spare kit in your stock, it will become obvious as you pull it apart as to what part is faulty. p.s.-There is more than one hole in the bottom of the master cylinder reservoir. Take a long wire and poke it through them into the barrel below. You will have to completely remove the top cover off the master to do this. Be careful to not rip the lip of the rubber cups on the master piston. (When you have the cover off the master cylinder and put your foot on the brake, a fountain of brake fluid shoots up out of the reservoir. To avoid this either drop the cover in place sans bolts or press on the pedal VERY slowly.)

Yes I have the fountain... Lol. Still haven't figured it out. Took all the wheel cylinders apart. They look to bein good operating condition. I figure there has to be an "internal leak" somewhere. It has to be in the master cylinder. Have had it apart once already. Just going to rebuild it... Even though it looks fine.

If you rebuilt the master cylinder and used a NAPA kit, you must reuse the old piston (with the new seal, comes off easy) as the one in the kit is too long. Made that mistake last week.