I have a 1987 F250 6.9L Diesel 4X4 and when I bought it the brakes were really soft. The issue was a bad master cylinder, which I replaced. After bleeding the brakes, everything seemed great. Next thing I know the rear drums are locking. I replace the rear wheel cylinders and shoes and everything seems great until I drive it a short distance and again the rear brakes lock up.
I have bled about a gallon through the system (bleeding it at each wheel and at the differential proportioning valve) and still the rear brakes lock up.
Some other scenarios that seem to impact the issue are the daily temp . . . the higher the temp the quicker it locks up. A load seems to push the brakes into locking up quicker too.
A few other pieces of info that may help with this issue is that if the brakes lock up and I leave it overnight, the brakes release and it is fine again the next day. When it is locked up, if I pry the drums off the wheels, the wheel cylinder posts are extended and will not retract until the pressure is relieved naturally over time or if I bleed the brakes and relieve the pressure.
I read about the Brake Differential Proportioning Valve possibly being an issue, but I cannot figure out 1) how to test if it is faulty? 2) Can it be rebuilt if it is faulty? (I am assuming that if I purchase this part at the junk yard, it's a gamble whether or not it's any better than the one I have.)
After many locked up issues and subsequent bleedings the ABS light began coming on when the issue occurred, this was not the case initially.
BYW - the emergency break is not the problem.
A few questions:
I have read about adjusting the shaft that enters the master cylinder . . . Can this be an issue for this model 1987 F250? I have not seen instructions for this procedure in my manual.
Any suggestions on how to test the Brake Differential Proportioning Valve to KNOW that it is the problem?
Any other suggestions regarding what could cause the brakes to lock up like this.
Thanks for your help . . . I am desperate!

 

Check your rear brake hose (rubber hose from frame to axle) and maybe replace it. I've seen where a brake hose can collapse inside and will either not left pressure in, or it won't let pressure out. Not sure if that's your issue, but it's easy to diagnose.

 

2X on what BigBlockF350 said. Also clean the star adjusters and relube them, I use never- seize for they won't rust up and quit working.

 

Thank you for your comments, I checked out the rubber brake hose from the frame to the axle and it looks okay, but next time I have this issue I will observe it closely to see if it collapses or anything. At the junction where the rubber hose connects to the metal hose running across the axle it appears that another rubber hose was connected that ran up to the bed. All that is left is a short piece of rubber hose, any idea what this is for or where it goes too?

I lubed the star adjusters quite liberally recently, but next time I am in there I will lube them again.

Thanks again for your help - KB

 

If the hose is collapsing and not letting fluid back out, you won't necessarily see it from the outside. The inside lining can collapse without any visual difference of the outside of the hose.

 

If you do not know when the last time the brake hoses were replaced it would probably be a good idea to replace them. When I bought my truck a couple of years ago that was one of the first things i did was replace all of the brake hoses.

Matt

 

There is a front and rear shoe for these drum brakes too and there isn't a real obvious difference between them you have to look closely.. one has slightly less friction material on it. If these are on backwords(smaller shoe should be towards front of vehicle) or you have both fronts on 1 side and both rears on the other the brakes will be very prone to lockup.

 

I know it may sound dumb, but how full is your master cylinder reservoir? If it's full to the top (past the full mark), then as the fluid heats up from stopping it'll expand but won't have anywhere to go. The point of least resistance for the expanding fluid is the brakes, and the drums take the least force too start to apply. They will start to apply when the brake fluid gets hot, and they won't let off until the fluid cools.

 

That short piece of hose you mentioned in post #4 that just goes up to the frame rail is the differential vent tube.

Sounds like good advice in here. Definitely check the shoe orientation, if that's all good I'd swap the rear flexible hose with new.

Will the brakes unlock if after locking up you crack open one of the metal lines at the rear T that is on the axle? If yes, you then know the issue is upstream of the T fitting.

Will the brakes unlock if you crack open the line upstream of the rubber hose? If not, you've now shown the issue is downstream of that junction, at which point I'd swap the flexible hose. If that does free up the brakes, keep moving upstream toward the proportioning valve.