I have a 1972 F100 4x4 with power brakes and a non adjustable brake pedal. The brakes have never been great but they got me by. A few months ago I was leaking fluid from my master cylinder so I bought a new one and put it on. I bench bled it first then put it on a bled the rest of the system. At first the pedal was soft but got harder after pushing it a few times. Problem is if I push the pedal more than a few times it gets very hard to push and my front brakes lock up. The only way to release them is to hop out and bleed some fluid from one of the front wheels. Turned out Oreilys sold me a master cylinder for 2 wd and I thought maybe that was the problem. I ordered the right one put it on and still have the same problem. It's bad enough I have blown 2 brand new 35 inch tires so any advice would be greatly appreciated. Also my truck is disc on the front and drum on the back.
Thanks

 

my bet is the push rod is too long.
i had the same problem on my 79 after replacing the booster.
ended up the pushrod going into the master was 1/8 inch too long causing the piston not to recenter when the pedal was released. this caused the front circuit to hold pressure.

 

We had problems with a stuck caliper causing similar problems on our 72 F250 Camper Special and then later on our 78 F250 Ranger. First we had to replace the RF and later the LF. On the Ranger I believe it was just the opposite. We still have the 72 F250 out on the farm.

Never had to replace a booster, but have replaced sever master cylinders. A couple of those were pure PIA's trying to get successfully bled out. But that was usually indicated by pedal to the floor with minimal braking effectiveness.

On our 78, I also had a bad proportioning valve affecting the front brakes but not the back. IIRC that was a soft pedal as well.

 

First look for me would be see if there is free play of the push rod contacting the piston in the master cyl. By feel and or by looking under the dash there should be 1/16 to 1/8 free movement of push rod as it goes forward to the M C piston. Just pressing brake pedal with your hand you want detect no effort until p-rod makes contact. If no free movement check to see if the bolt the eye of push rod goes thru MIGHT be eccentric for adjusting resting pedal height. If pedal cannot go back any farther it could be holding MC piston slightly applied.