I have a 1978 20R with a 5 speed. My front calipers are seizing up when driving, if I drive on the freeway in 2 miles I start to smell the breaks and the truck starts to pull hard, as if it was dragging or towing something. If I drive around town it takes about 4 to 5 miles for it to happen. I have to release the bleeder nipple on the calipers in order to release pressure on the calipers. I have never seen or heard of anything like this before, all internet searches point towards ABS or electronic breaks. I have had the break booster rebuild, the master cylinder is brand new and the 2 front calipers, pads and rotors are brand new. Bled the system from the back passenger side first then worked my way around to the front driver side. The break peddle is nice and firm but when driving the peddle starts to firm up and then the calipers start to clamp down on the wheels.

Any one have any ideas about this?

 

Something we need to know is, when you open a bleeder on one caliper does it release the pressure from both front calipers or do you have to open both bleeders?
If you need to open both bleeders to release the brakes you should repeat the test but crack the fitting for the front brakes at the master cylinder. If that doesn't release the brakes replace both front brake hoses, bleed and repeat the test.
If one bleeder releases both front brakes you need to repeat the test but pull the vacuum hose and check valve from the booster to see if the brakes release. This would indicate that either your booster is faulty or the pedal adjustment is off making the booster applying pressure to the master cylinder whenever there is a vacuum applied. Make sure there is freeplay in the pedal linkage when the pedal is released.
If that doesn't release the brakes your pushrod length between the booster and pushrod may be adjusted too long leaving the brakes slightly applied even when the pedal is released.
If you remove the nuts holding the master cylinder to the brake booster (moving the master cylinder away from the booster) and the brakes release it is 100% certain that the issue is with pushrod/pedal adjustment or faulty booster applying pressure to the master cylinder. If the brakes still do not release it is 100% certain that the issue is trapped pressure in the brake hydraulics either from blocked hoses/tubing or the master cylinder has a blocked port.
The change after driving is likely related to heat building up from the dragging brakes causing the rotor to expand and the brakes to drag harder.
I hope this helps! Good luck!

 

Also keep in mind that every time you cook the front brakes you are reducing the lifespan of the calipers. It will harden the seals and eventually lead to dragging brakes again or leaking calipers.

 

Sounds to me like the pedal adjustment. Bear in mind, I am NOT a brake guy. I know just enough to be dangerous

Another little detail I noticed, that' may be nothing, but hey, ya never know: You didn't SAY you bled out the LPSV. If it's stuck sending all the pressure to the front brakes, that could be a problem. Bleed out the LPSV last, after all the wheels. I've also found that disconnecting the bar that connects down to the axle and cycling it through it's entire throw a few times can free up a lot of troubles. Try cycling it before you bleed the LPSV, so any gnrrrr (guh-nerrrr: icky, nasty, gunk clogging things up) that breaks loose IN the LPSV gets flushed out when you bleed it out.

Having said all that, if the LPSV isn't the trouble, I'll say that the other guys have a better, more complete guide to all this. I'd follow their advice before you listen to any of my rambling.

Good luck to you, whatever the trouble is.
Pat☺

 

I'm 99.99 percent sure this is the problem. When I installed the brake booster the screw that holds the peddle adjustment is to long and is pushing the peddle forward I had to jam in there to get the peddle connected. The first test drive after installing all the hardware and bleeding there was a lot of air in the system half of the peddle was dead and the breaks would barely grab at the bottom, could not tell the difference in the vacuum pressure. The second time I bleed the breaks I felt the vacuum pressure after a few miles and especially on the freeway cause vacuum was being applied tot he breaks all time whit the break booster being pushed in a bit.

I had to cut off 1/8 of an inch from the end of the bolt so it would clear the break peddle completely. Marked the cut off with white threadlock.

On the reinstall I twisted the adjustment screw in until the pin would slide in freely and the break peddle would be lose and able to jiggle freely. The break booster has no pressure from the peddle right now. It took me 6 tries to install and uninstall the break booster till I got it right, tomorrow I will bleed the breaks and go for a drive around the block.


To answer your question @akwheeler one bleeder releases both calipers. I did not want to put extra stress on the master cylinder pipe screws cause they are warped. I think the previous owner used an adjustable wrench to open and close them. They are in really bad condition I don't know if I will be able to retighten them again to be honest.