77 F100 2wd, 4.9L I6. Had a bad proportioning / distribution valve. Got a replacement at the junkyard and that allowed me to get fluid to the rear drums. Rebuilt the MC. Primed / bled the brakes and life was good.

After driving about 100 miles, the rear brakes started to lock down. It is like they autoadjusted too much. I backed them off using the adjustment wheel. In less than 10 miles, they tightened back down.

Why would they start tightening up? Is it possible I've got the wrong prop valve? I didn't check the donor vehicle to see if it was drum or disc.

Anybody had this happen??

 

Chances are you have a master cylinder problem, or the push rod coming off the brake pedal is a bit too long. If its power brakes, the end of the rod as it goes into the m/c is the adjusting point. Ive seen this a few times where my customers had used the wrong fluid-ie not brake fluid, and the brakes would begin to apply on their own. Did you use any solvents when you cleaned out the master cylinder that may have swelled the new seals?

 

2x on the rod adjustment...

 

^^ bingo^^

 

if it was the rod why would it only be locking his rear brakes? I'm thinking you need to take the shoes off and take a good hard look at the drum hardware. The shoes may just be hanging up for some reason. make sure the springs and pawls are all where they are supposed to be. Having a disc/disc prop valve might cause weird rear brake balance but it shouldn't cause the rears to drag... the disc prop valves have less residual pressure than the drums.

 

I will check both the rod and the drums. Like I said, I am stumped. I didn't use any solvents. Just brake fluid.

 

Pulled the drums. Rear drivers side was over tight. Yesterday I had loosened it and it tightened back up. The ratchet mechanism in the autoadjust isn't right. The little piece of metal doesn't go into the teeth. It adjusts in both directions and I know it shouldn't . Passenger drum is correct and I can't quite figure why one is right and the other isnt.

Is it possible that's the problem?

I'll still check everything else, but at this point I am looking for stuff that seems wrong and fixing it before I move on to the next subsystem.

 

Not the clearest picture of the adjuster but is it like this?

 

Yes, that looks like the passenger side. See the little metal tab that drops into the teeth of the adjustment wheel? On the drivers side mine sits lower and rubs against the adjustment wheel but doesn't engage. I'm 99% sure I've got the right adjustment mechanism, why is it different than the passenger side (looks lik your picture).

 

It could be I had a Jeep that had similar problems and it was as simple as I had two left hand adjustment screws instead of one of each. Not saying that's what's going on in your case but it sounds like it could be something that simple... Just a side note I had checked all my pads lines and cylinders and changed my master and flushed all the fluids before I found the dang problem

 

I'm not sure on these specific drums but on mine there is a small metal tab that slides into the drum housing its a pain to get in position but if it is not fully seated it can keep the adjustment tab from engaging the adjustment screw

 

Possibly it was replaced by a PO with the wrong adjuster, who knows but they should be alike. Maybe just disconnect the selfadjusters, re adjust the brakes and drive it a few days and see if thats your problem

 

OK, at this point I'm just talking to myself, but I think the blinding light of the obvious just flashed.

The metal tab isn't supposed to engage the wheel normally. Only when it it time for the self-adjuster to adjust. When the pad wears the excursion is greater. When the excursion of the pad is great enough the spring pulls on the adjuster lever enough to raise it up. When it raises high enough it engages a tooth on the adjustment wheel. When you take your foot off the brake and the pads relax it spins the adjustment wheel one tooth.

the tab shouldn't engage the adjustment wheel normally, only when it needs to bump the pads out a little bit.

Did I get that right??

 

lol I think they adjust whenever you hit the brakes in reverse. A friend used to disconnect them on his trucks and just adjust the brakes whenever he changed the oil, said the self adjusters wore the shoes out prematurely. Granted, he was a farmer that used his trucks in offroad muddy conditions and did a lot of backing up down the well roads too.
If I'm not mistaken the adjuster tab should make contact with the star wheel at all times, remember, one click on the wheel isnt going to move the shoes out much, sounds almost like yours is adjusting everytime you hit the brakes, if thats possible.

 

The metal adjuster arm turns the adjuster as needed. The arm does rest on the tooth portion of the adjuster. Can you post some pics?