Rear brakes dragging over time

It seems I am having an issue where my rear brakes are slowly locking up. Drove to work yesterday (24 miles) with no problems, on the way home they slowly began to lock up. I had to pull over and loosen the self adjusters. They are not self adjusting to lock up so I am suspecting the hydroboost. What I have is:
Lincoln HB, stock Bronco master cylinder, stock Bronco pro valve, stock brakes front and rear, Lincoln dual return pump. Everything is new except the HB. I also installed an inline PS fluid filter. This didn't seem to happen the first couple of times I drove around. Suppose that filter plugged? Any help is appreciated!

Old hoses swelling and not letting fluid return?

Maybe the brakes were adjusted too tight. They need a little space for expansion when they warm up.

What was it plugged with?

Interesting.....hmmm...guess I need to do the filter thing sooner than I thought...

If you mixed and matched a master cylinder with the hydroboost you might have a hair too much preload on the master cylinder. Just enough to cover the transfer port and not let the brake fluid to breath back into the reservoir.

Make sure the adjusters are set right. Backing them off is not the correct fix if they are correct when cold. If this happens again, crack the bleeder screw and tighten it back up. If that fixes the issue, you have a hydraulic issue and not an adjuster issue.

Well that's a first for me. Never heard of a plugged power line in a booster causing only the rear brakes to start tightening up.
Interesting.

But no matter what, hopefully it's fixed for good. Good call on the filter.

Paul

Back pressure on the return line could have been causing the booster to not retract all the way. With no place to go the fluid will move whatever is easiest to move in the system. My concern would be what was the material in your filter. especially if enough to block it up. Was it black, whitish ect? could have a seal in either the Hboost or the steering box going out. The pump is not likely to have any seals that would get into the system as usually the only seals it has is the shaft seal and the reservoir seal. Only other thing I could think of is a piece of paper towel or a rag was in your new pump. A rag would be a sign of a builder that doesn't care as paper towels are more the standard for hydraulic use especially in the world of pumps.

Are you running a T fitting in the hydra boost return line and the return of the power steering line. Any restriction in the return line with a tee fitting setup will apply back pressure to the Hydra boost causing drag. The net big thing is the rod between the hydra boost and the master cylinder. If the rod is too long it will apply a little pressure to the master cylinder piston dragging the brakes. A quick check is when the brake is dragging, jack up the rear on jack stands and put the truck in neutral and try to rotate the tire. If its dragging loosen the 2 nuts keeping the master cylinder and the hydra boost together. Loosen the nuts to be able to pry the 2 apart an 1/8" now try to turn the tire. If it now turns free then you either need to make sure the peddle returns all the way back to release pressure or the rod between the hydraboost and the master cylinder is too long.

Maybe you need to back off the adjusters a bit more. Once you use the brakes they expand.
Also, I just had my PS box rebuilt, and the guy at the shop recommended no filter. He said it creates more problems than it fixes.
He did recommend purging the system with at least two gallon of new fluid. PS fluid really is pretty inexpensive.

While taking the HB and loosing did not resolve this could be because the rear drums may have a residual pressure valve. Usually built into the M/C port. I would take the M/C and loosen the bolts enough to slip a shim on place - a body shim for example - and tighten the bolts back up. Then repeat your drive test. This will remove or reduce any issues with the pushrod length or mounting. Just dragging the rear brakes is pretty odd I bet the front are dragging too just to a lesser extent.

If this helps you can shorten the pushrod, make a nice looking shim or just slip some washers under the studs. My Dad had a Heep I made this adjustment and it stopped dragging the brakes with HB.

Your front brakes are plumbed into the port closest to the firewall? When you get back from your trail ride and the brakes are dragging if you open the port on the wheel cylinder does brake fluid dribble out or does it squirt out like its under pressure. Make sure your wearing safety glasses. Brake fluid and eyeballs don't mix.

I am watching this!

Can you post a pic of your brakes with the drum off, master cylinder and booster?

A residual pressure valve keeps a little pressure in the line (I have heard 2# and 10#) to keep the shoes extended so the pedal does not travel so far. As RustyTruck said after a ride with the brakes dragging loosen the fitting at the master cylinder until it leaks a bit then go for another ride. If they are free then you know the master is the problem. If not then you have mechanical issue in the drum I think.

The RPV's, when working properly, don't really keep the shoes out and in contact with the drums. Ultimately the self-adjusters (or more likely the manual adjuster that works on the truck!) do that job.
The 2lb versions are for disc brakes and the 10lb version is for drum brakes. But the springs alone in a drum setup are strong enough to push fluid back into the master each time the brake pedal is released. It might slow things down a bit, but it should not stop it.

That said, it's doing that only if it's working properly. If one is stuck, or has some gunk blocking the port, it could keep fluid pressure in the lines.
The original and main purpose of the RPV is to keep fluid in the lines where the master cylinder is low mounted, or in some old setups, even below the level of the brakes. In the '20's and '30's, many master cylinders were mounted down on the frame rails, below the driver's feet.

Most new and rebuilt masters no longer even come with RPV's because the last low-mounted master was many decades ago.
So valve or not, on your basic single cylinder drum brake setup the shoes should always pull back to the point that they are hard up against the anchor post by the wheel cylinder. In our case, at the top of the drum.
When things are working as they should, only the adjuster keeps the shoes within reasonable striking distance of the drum. This is why your pedal travel gets longer as your friction material wears, whenever the auto adjust feature is not working properly.
As many of ours don't.

If after applying the brakes, your shoes are not able to pull back down fully to the anchor pin then Tim (at least within a couple of seconds) then something is holding pressure in the line(s).
As often as not it's a deteriorated soft line (looks fine on the outside, gooey mess on the inside), but I don't see why it can't be a master cylinder OR a proportioning valve either, if both rear brakes are holding.
Can't see how one of those would effect only one wheel, but that happened just a couple of days ago with someone. Should not happen though, so perhaps one brake was adjusted farther up than the other, so when the hydraulic pressure was just barely applied, only the tighter brake got hot.

Paul

I forgot if you said Tim, but most of this stuff is brand new, right?
Even new hoses can be faulty. And so can a master cylinder, wheel cylinder and proportioning valve (if so equipped?).

So what is new and what is old still in your brake system?
Prop (combo) valve or not?
Braided or rubber hoses?
Checked and made sure that the shoes were oriented correctly (short shoe to the front)?

Sorry if you went over all that already. Don't remember.

Paul

It may be the picture angle but it doesnt look like the brake shoes are sitting squarely on the pads on the backing plates. the spreader bar for the parking brake isnt sitting right. I think the shoes are wrong. You have one brake adjusting wheel crammed up tight and it shouldn't be that way.

what size drum are you using? what year is the rear axle from?

look at this diagram.

It is hard to see the bots behind the axle but these pics are good (your rig looks REALLY nice and clean too!).

I would adjust both adjusters to something similar like you can see two threads. Then drive it around until they start to drag. Remove shoes and see if the adjusters moved. If they did I suspect something in the set up of the hardware in the drum is incorrect. If the brakes drag but the adjusters did not move then you have a hydraulic problem.

You said you had no brakes this last test drive. Are your front brakes a problem also? Are they disc?

Did you use a rebuild kit or replace it?