The Stovebolt Site Forums The Parking Lot Tons 'o Fun Front tires lock up

89 3500 dually with a welding bed on it, under hard breaking and on wet surface the front tires lock up. Kinda spooky when you drop off a ramp and try to stop, puts crap in your neck. There is a valve over the rear axle with a arm and rod attached to the axle with brake lines running thru it, is this affecting my brake balance. New drums, rotors, pads and shoes, calibers, hardware and wheel cylinders, hydra boost system I just want good brakes, I do a lot of city driving and leave plenty of room between other drivers but sometimes quick stops are necessary,

Yes. That valve at the rear axle increases the rear braking acording to the load. Did you replace the front brake hoses? They were bad on this model truck about swelling up.

Yes the hoses have been replaced along with everything up front, when this happens seems like the rear brakes just free wheel. It would be easy to just reroute the brake lines eliminating that valve, might be an easy way to check it out. Just want to get it fixed before the snow flies. Thanks for the quick response.

Hy Spareparts2, did you do the brake work or did a shop do it? The whole thing should have been bled after all the hydraulic parts had been replaced. If you have a good shop you could have them put pressure gauges in the lines at the calipers and wheel cylinders and see what kind of readings they get, there may be something wrong with the valve over the rear axle, not allowing enough pressure to the rear brakes, hope that helps.

3B makes a good point. With Hydraboost 1/2 of the system may not be working & you dont feel it in the pedal.

I did the brake job and yes it was bled with a pressure pot type bleeder, actually it was flushed out with about 1/2 gal of new fluid.This problem is irratic, about the time you get to trusting it it bites you. Thanks for the tip on the pressure gauge, I have a set of gauges out of the race car, if bypassing the valve does not solve the problem I will plumb in the gauges. Thanks for all the help, we will get this problem solved.

I did the brake job and yes it was bled with a pressure pot type bleeder, actually it was flushed out with about 1/2 gal of new fluid.This problem is irratic, about the time you get to trusting it it bites you. Thanks for the tip on the pressure gauge, I have a set of gauges out of the race car, if bypassing the valve does not solve the problem I will plumb in the gauges. Thanks for all the help, we will get this problem solved.

Don't bypass the proportioning valve, it is put there for a purpose. If everything else doesn't give results, adjust the link on the valve. They are supposed to give more rear braking w/ heavier rear axle loads, less when empty. Perhaps the valve got bumped or?

I know better than that, thanks for the advice thou. didn't have time to check it out today wasn't feeling to whoopie only lasted 4 hrs and came home.

Rerouted the rear brake line and have been waiting patiently for some wet roads to really check it out, well it rained all day and everything seems to check out good. Something I failed to mention was that I have air bag helper on the rear to compensate for heavy loads, that probally gave me some false readings by not letting the truck drop down under loads. I got on the brakes hard several times today and the brakes felt good with NO front lock up. Thanks for all the help.

Hy Spareparts2, thanks for letting us know. When you say you re-routed the rear brake line, how did you re-route it, and why did you think that might help?

Well that was kinda the easy part, the rear line goes to the valve, then out of the valve in a U shape steel line to the bracket for the rubber line to the rear axle. Just removed the U shaped line and straightened the rear line up and screwed it into the rear brake hose, bled it and tried it out, good brakes now. I guess the valve is GM's feeble attempt at anti lock brakes on trucks, but on my truck with the heavy load and air bag helper's it just was'nt working good for me.