I am completely flummoxed. I have an 1988 F-350 4x4 Crew Cab with a 460 I've had since brand new. For some years I was driving a new Super Duty so the ol' 88 didn't get driven much. I started driving it more last year and noticed the front brakes were dragging. Okay, the calipers were original, so I'm thinking maybe some rust in the calipers was keeping them from releasing. So... I installed new calipers and new pads, lubed the surfaces, etc. Still dragging. Replaced the rubber brake lines. Same issue. I installed a new master cylinder, new booster, did a complete change of fluid, bled the system. The pedal is a little soft at start up, then it progressively gets harder and the front brakes start dragging. NAPA said it sounded like the master cylinder was the problem. Since it was still under warranty, I replaced the master cylinder AGAIN. Saaaame problem. I am out of ideas. New calipers, new booster, 2 new master cylinders, new rubber brake lines, flush and bleed. One weird manifestation is that I seem to be losing brake fluid. Not a lot, and I'm not seeing any fluid leaking anywhere. I'm trying to ignore the niggling feeling that the booster is defective, but otherwise I'm out of ideas. But even if the booster is defective, why would that cause loss of fluid, a hard pedal, and not allow the calipers to release? I believe the master cylinder would have to be bad to cause brake fluid to get sucked into the booster, and I can't quite believe I've had TWO bad master cylinders. ANYONE have an idea??

 

Rear wheel cylinders could be a source of your fluid leak. A shoe can soak up alot of fluid and the drum sling it away. If you have RABS, don't forget to check that as a leak source. It's down on the rail below the master cylinder. As for your sticking fronts .... if you're sure you have no binding up issues with the calipers to the mounts, or pads fitting tight in their slot and causing them to bind up, I'd start looking hard at the porportioning valve.

 

There is no proportioning valve on these trucks. The front brakes are a straight shot from the master cylinder. The rear brakes are modulated by the RABS hydraulic module located on the driver side frame rail, already identified by a previous post.

The lost fluid could very well be the rear wheel cylinders. Those drum brakes can mask a leak for a long time. Highly suggest to pull the drums for an inspection. The number of parts you have gone through is amazing. A test to help ID the root cause of the dragging calipers is to drive the truck until the symptom returns, when it does crack one of the caliper bleeder screws to see if the calipers release. If they do it's a hydraulic problem. If not it's mechanical. I know you have replaced several parts and lubed the sliders, but you need to figure out if it's mechanical or hydraulic.The one piece left, the vacuum booster, I have never seen cause this issue but there is always a first. Process of elimination before emptying your wallet some more.

Always tough to diagnose multiple issues.

 

Around the same time as all this other stuff was going on, I had a rear axle seal leak. I replaced the seal (on that big Sterling axle, that was a job!) and while I had it all torn apart I went ahead and replaced the shoes and wheel cylinders. I suppose one of the new cylinders could have been de fective and is the source of my fluid loss. I'll have to check that. I'll take a look at the RABS, but I'm not seeing any brake fluid anywhere when the truck sits. Thanks for the reply!

 

Yeah, I didn't think there was a proportioning valve, except maybe mounted directly to the master cylinder, which of course would be replaced at the same time as the installation of the new one. The front brakes drag hard enough to start smoking after a while. I hadn't thought to crack the bleeder and see if the brake releases, and I haven't been able to isolate with any certainty whether it's mechanical or hydraulic. I suspect it's hydraulic for this reason--- if I let the truck sit for a few hours, the brakes eventually DO release, and it seems to me that if it was mechanical they would stay stuck. But what I can't figure out is what is preventing the pressure in the calipers from being relieved. If I crack the bleeder and the brakes DO release I'm still at the stage of wondering what is causing the pressure to build up and not back off. It's a real head-scratcher, I'll tell you!

 

Interesting .... I actually went out to my truck to check this. There is NO proportioning valve ... lol !!! Was confusing it with the small "T" block that splits the fronts. Anyway, my apologies.

Only other things that comes to mind with this, in the event that your issue is in fact a hydraulic one, is the brake booster rod. If adjusted too far out, it wont fully release the master cylinder when you get off the pedal .... but may after it sits for a while and looses vacuum. Additionally, how about your pedal mechanically - does it move freely down/up, or is it binding near the end of its return?

 

I just had the exact same symptoms and problem except the booster was the last thing I replaced and it fixed it. If the seal is broken inside the master cylinder, fluid will get sucked in the VACUUM booster. That may be where it is going. I would loosen the booster and check for fluid. If so replace MC again. If no fluid, adjust the booster rod in a full turn or maybe two and see what results you have. If it still does it, my money is on the booster. If you hear any hissing when you step on the brake it is almost assuredly the booster and I wouldn't bother with the rod. Sandy

 

check vacuum with gauge and engine running with booster intact, see if your needle ticking and moving? Remove booster hose and plug on vacuum side, see if vacuum higher and needle tick on vacuum gauge goes away?