Daytrepper

Hi all, I am a bit stumped on this one.

Just got my 1993 D250 back on the road after restoration. I have about 100 miles on it now.

The rear brakes are intermittently dragging. They will release fine until you make 3-4 stops, then they start to drag....to the point that the pedal sinks a couple inches, and stays down far enough to keep the brake lights on. Wait a few minutes, and they release, then you can make 3-4 more stops without them dragging, then they start again. Pedal returns to normal position and brake lights go out.

The entire braking system is new, except the proportioning valve---because I simply could not find a replacement.

RWAL hydraulic unit is new.

It appears the proportioning valve is working, as the plunger moves in and out with the brakes applied, like it is supposed to.

I found the "new" master cylinder leaking, so I replaced it; also, just in case, I replaced the rear brake hose.

It seemed better, but after a 20 mile total drive to town and back, the rear brake drums were around 150-160* temperature, and I could feel a slight drag stopping and starting.

The rear brake adjusters and parking brake cables are completely backed off. If I adjust them up at all, it gets worse. I have pulled the drums and made sure everything was installed correctly, and it is.

I did a few panic stops with it as a test and the rear brakes lock right up; RWAL then kicks in and releases them; this seems more like a proportioning valve issue IMO;

Any ideas? I can probably source a used proportioning valve just to try; which is about the only idea I have left.

kensmachine

My trouble with dragging brakes was the parking braking brake cables you can only get one length cable on the left side and it's about 4- 5" too long for a 2WD truck all the sources list the 2WD and 4WD as the same cables left or right. Ken

j_martin

If the pedal is hanging, then the return spring is off, weak, or something is wrong with the master cylinder, brake booster, or pedal linkage. If the pedal doesn't completely return, then the brake fluid is not allowed to return to the MC.
I have found that most new rear brake shoes for this truck are missing the top flat for the parking brake lever. That causes poor action, and can bind.

Daytrepper

From what I can see the pedal is not returning. It will come back to a certain point on its own quickly; then, stop, and slowly return to top position---so that tells me the front circuit is returning but the rear circuit is sticking, as that is applied first in the top few inches of the brake pedal; once its all the way back up, no more brake drag. I can pull it up with my foot----its maybe sticking down 2 inches---and it will stay there, brakes are then released.

Not sure about the flat for the parking brake lever. Thats something else to check.

Booster was replaced during restoration as well; if its bad I'd be surprised, but with the way parts are today you never know. I would think the problems would be worse if it was bad.

There is a place that rebuilds the proportioning valves for these things. Might send that off, considering, it is 26 years old, which means it has 26 year old O-rings in it; if something is binding or sticking it would restrict the return of the rear brake fluid.

j_martin

Even if the rear circuit was completely plugged, the master cylinder should return to rest quickly. If it didn't you couldn't "pump up" the brakes when a circuit fails or is out of adjustment. When it fails to, the fluid is prevented from returning to the MC>

I replaced a booster a few years ago, and several boosters I got from NAPA all failed because the retainer that controls the return limit failed, knackering the stop light switch adjustment. One from O'Reilly worked. I can't remember, but I think some pedal drop was involved as well. Without going into the diagrams, I don't know what drives pedal return for sure.