I bought a '90 F150 4wd from my boss about a month ago. He is the original owner and I have worked with him for over 7 years, and I trust him. I drove the truck for over a week before deciding to buy it, and in that time, the truck drove fine.

Then suddenly, it developed a brake problem. The problem is that when braking, the truck instantly shifts to the left (doesn't necessarially steer to the left) and the driver's front tire locks up. This happens with regular braking pressure even at speeds as low as 10mph. It is greatly exaggerated at 30mph+ and is unsafe to say the least at higher speeds. Forget about safe braking in the wet. I have not been driving it much since this happened because if I had to make a panic stop, the damn thing would try to swap lanes and could cause me to hit someone.

Logically, I thought the cause was a bad caliper on the passenger side. So to be safe I went ahead and replaced both calipers. No change. I'll go ahead and list all the things I've done to try to troubleshoot this. I'll also note that the rear brakes are brand new and functioning properly. I can step on the parking brake pedal to test the rears and they slow the vehicle with no pulling to one side or the other.

Things I've done to try to fix this:
-Replaced BOTH calipers
-New Pads on both sides
-Machined rotors
-NEW rubber hoses on both driver's and passenger's side

Both calipers bled fine with plenty of fluid flow, and both calipers are grabbing while testing with the front end up in the air.

It's getting to where this 1k truck is costing me more money and hassle than it's worth. I'm kind of at a loss as to what is causing the problem. The front suspension checks out out. Radius arm bushings, etc all look good. There is some slack in the steering box which I plan on adjusting out.

The hubs are in good shape, both rotors turn freely while the front end is up in the air. The truck has rear ABS only as far as ABS goes.

I'll try to supply any information asked for. Thanks in advance for any advice or tips on what to try next.

Truck Specs:
-Automatic trans
-351w
-31x10.5 tires
-Short bed

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Check to make certain that the calipers slide freely on the brackets. If they don't disassemble, clean and deburr as req'd.

Check and adjust the rear brakes. If you're not feeling the pull in the steering wheel, there's a very good chance that it's coming from the rear. That doesn't explain the LF lockup but chec it anyway

 

Well, I'm a dumbass.

I just drove it around a little bit more and paid some more attention to it. It is coming from the rear. I never really let the tire lock enough to pinpoint that I guess because I didn't want to flat spot the tires since they are new.. and the I haven't fixed the window regulators yet so I couldn't roll the windows down and I guess I was being fooled and thinking it was the front.

So now that I know it's the rear, I guess the first logical thing to try is the RABS sensor and maybe the prop. valve but I will probably do the RABS first since it is $20 and easy to replace, and if that's not the problem I can always return it.

Any other ideas? I will post back.

 

Pull the drums and look at the shoes, wheel cyls, adjusters and springs. something is FUBAR and it's not likely to have anything to do with the RABS.

My bet is that the RR shoes are not extending enough to engage the drum. Likely causes include a rusted or frozen adjuster or a broken spring. It's also possible that you just need new shoes. It's not likely to be a blown wheel cyl but if it has leaked enough brake fluid onto the shoes they simply may not have any friction left. It's really a simple inspection/diagnosis. Try to pull the drums without backing off the adjusters. Sometimes it's just not possible but you really want to see the condition of the adjusters in their natural state of adjustment if you can. If you've not done brake drums before, take a picture withthe drum off before you disassemble the rest, it'll help you to get the springs and hardware re-assembled correctly.

 

Have you used the e-brake ever?

Mine developed a similar problem after I used the e-brake while parking on a steep hill to keep the load off the park pinion. Since on auto cars the e-brake is rarely or never used, the first time after years is also the last. The pedal popped back up fine after, but the cable was rusted to hell and it didn't release on one side. It wasn't really noticeable while driving, but even moderate brake pressure would make the back wheel lock up and the truck to pull. I checked a bunch of things including the ABS system before thinking about the e-brake.

 

Yea I use the parking brake all the time, but I will definitely check it this weekend while checking the adjustment of the rear brakes.

 

Could be a leaking axle seal getting grease on the brakedrums inside..

Instant lock up when that happens..

While you are in there change the seals..
They probably need it anyway..

...zap!

 

The axle seals and bearings were recently replaced.. no rear end lube present in the drums.

Update: I changed both rear wheel cylinders and that fixed the pulling problem.. the passenger side cylinder was siezed and couldn't be moved.

Now though, instead of just one rear tire locking up they both do which I guess is better, but it happens even under light braking, so I guess either the RABS sensor in the rear end is bad, or the RABS unit itself is bad.

Which is more likely to be bad? The ABS light on the dash is NOT on.

Also, a friend of mine who is more familiar with these trucks than I am said some of them had a valve/sensor that would decrease brake line pressure to the rear wheels as the load in the bed decreased.. is anyone familiar with this system and is it something that fails often?

Thanks for the help so far..

There is an ABS test port under the hood.. what's the procedure for testing it using this port?

If it looks like it's going to be expensive to troubleshoot the ABS, I'm just going to buy an aftermarket manual proportioning valve and adjust the pressure out that way

 

Pull codes. Just becuase the light isnt on doesnt mean it isnt throwing a code. I vote for the rear ABS sensor.

And the pressure reduction thing is crazy talk.

 

could be master cylinder too the seal on the plunger could be leakin and givin more pressure to the rear then the front thats a nice truck tho got one just like it with 302 4spd granny tranny 6in suspension lift 3in body 35/14.50 boggers gettin a full rebuild when i get home goin carbureted tho buildin to a 347 stroker your body is in way better shape tho lol