If the hoses don't fix the issue, replacing them is a good thing anyway, the next time they lock up loosen the 2 nuts holding the master to the booster and see if that releases the brakes.
As a test you can install 2 thin washers between the booster & master to see what that does.
My guess is the rod between booster and master is still to long and not letting the master return all the way and allowing the fluid to return into the reservoirs.

Oh are you sure you have the right lines hooked to the right ports on the master?

What lead you to replacing the master and was there any other work done on the brake system?
Why could you not bleed the stock system?
You bench bleed the new masters?
After that how were you bleeding the system? Gravity, 2 person, pressure bleeding or all 3?
Dave ----

 

This had more replies than i expected to get in a night. I've had the rod adjusted all the way in at one point with no change so i highly doubt that its the problem. I couldn't bleed the system because the master was allowing air into the lines. It went away with the replacement master. The reason i don't put a stock master back on is because during bench bleeding I've had multiple turn out bad, most not pushing fluid out one port or the other with no change once installed on the truck. After installing I've been 2 person bleeding the rears and using a brake bleeder tool on the fronts. I tried 2 person bleeding the fronts thinking the way i was bleeding it could have been the problem but that didn't change anything.

 

Air will not cause the problem you're have, actually the opposite.

Adjusting the rod all the way in doesn't mean anything if you have the wrong master. the cup in the piston could be 1" different. but that doesn't mean you aren't getting it adjusted correctly, it's just a concern.

you may want to recheck a correct master or the bleeding system you're using . masters leak fluid and they can have insufficient pressure but it's nearly impossible for them to pull air , they're a flooded piston there is no air to take.

 

Did you reset the proportioning valve when you bled them? Just a thought.

 

I can try to warranty the three different brand new masters I have right now, during bench bleeding two dont put fluid out the front port and one doesnt put fluid out the rear port. I just know that forums on this site had said the master i put on could be used, but I did have to switch the lines around.

 

I didnt know you could reset the proportioning valve, I'll have to look into that. On another note, I replaced both front brake lines with no improvement, and removed the vacuum line from the booster also with no improvement. I ordered a new proportioning valve from Bronco Graveyard because after the booster it'll be the only thing except the rotors that wont have been replaced for the front brake system.

 

The proportioning valve won't cause your problems. I shouldn't say it's not possible, but highly unlikely.

If you switched the lines around that could be it. the drum brake fitting has a residual valve that's designed to hold some pressure temporarily on the rear drum brakes. putting that on the discs isn't going to work well but it should still slowly release. you really need the correct master.

It's inconceivable you got 3 bad masters. you're not doing something right. are you using the fittings and tubes to direct the fluid back into the reservoir ? this is a must. are you making slow strokes the full length of the piston travel ? sometimes it takes a while but it always works.

If disconnecting the booster didn't fix it then it is likely the master or the rod adjustment.

 

I don't know if you would actually call it a reset, but when you bleed the fronts, you have to pull out and keep it out while bleeding a little tit on the from of the valve. I use a small pair of vise grips.

May or may not be your problem, but thought I would throw it out there.

 

I replaced the master with one off my dads 76 temporarily. We know that his is good. we still seem to be running into the same problem, and i messed around with the rod a bit more still with no noticeable improvement. The new booster will be here tomorrow morning and I'll update you at that point. I'm just frustrated with not being able to get these brakes to work.

 

Back in 2017 I swapped to a modern translucent Brake Master Cylinder. After researching on our forum, I took Ultra Ranger's advice and sourced an used one from my local junk yard. Got mine out of a 1994 Mustang with the six cylinder. Prior to installing, I took the time to dissemble and thoroughly clean it. I do like the ability to see my brake fluid level without having to remove the metal lid on the cast iron master cylinders.
What is important. According to Ultra Ranger, you must match the original OEM brake master cylinder piston size with the new master cylinder. I don't recall specifically, but believe my 1977 F-150 4x4 W/400 original brake master cylinder had a 7/8" piston. And after reaching what Ultra Ranger mention on NAPA's web site, I confirmed I could get a 7/8" piston brake master cylinder off a 1994 Ford Mustang but it had to have the six cylinder, not the V8. Mustang V8s used a larger 1 and or 1 1/8" master cylinder piston. And as important, I wanted the brake lines on the same side as they were originally and an Ford OEM part. I can't guarantee this will solve your dilemma, but my gut feeling say it probably will solve your brake pedal issue.
Check the Brake Master Cylinder you installed to see what piston size is shown on it, more than likely it's too large.

 

According to the Napa website, the bore for my truck is a 1" and the bore off the 1990 was a 1 1/8" so that may have been part of the problem. But even after replacing the master with one we knew was good off my dads truck we ruled out that as the problem. I was able to get one of the new ones to bleed, so I put it on my truck and the front brakes are still locking up. I had to take a few days break from working on this truck to address some other projects I had going on, and because im rapidly blowing through my cash.

 

For anyone in the future who happens to have the same problem as I had, I replaced a warped rotor and the brakes function as designed now. I've been daily driving the truck for a week now with no brake issues whatsoever.

 

Thanks for posting up your Fix.