roughstock

I have a L8000 pulling a dozer trailer. The brakes are VERY sensitive when applied and lock up at times. Comperssor is good, have good air pressure. No leaks. New brakes, new slacks, have need adjusted.,
Do i have a bad proportioning valve? or is there a problem with trailer?

Hvytrkmech

What year Ford?

roughstock

1997

Hvytrkmech

Which brakes are locking up, (specifically)? Trailer only, tractor only, both or any single axle on either the truck or trailer? Or all axles?

Hvytrkmech

It could be the treadle valve,(foot valve). Ford did some crazy stuff with their braking systems. Does the tractor stop good by itself or do you have to stand on the brake pedal then all of a sudden it brakes hard?

roughstock

I'll check tractor by it's self. Never been unhooked from trailer. I'll try it on another truck. So far it's only been trailer axles locking up.

Hvytrkmech

any updates?

NE frmhnd

I've had trailer brakes that would lock and let go under medium to heavy braking. They would about put you through the windshield several times as you tried to stop normally, had to really put the air to the system to stop. Usually locked the trailer brakes in the process. Only when empty.

Turns out it was improperly adjusted brakes. The shop was tightening the trailer brakes a little lighter than the tractor. The anti-lock system was reading the difference as the trailer brakes locking. Adjust your tractor and trailer brakes at once, and back them off the same amount. I said something to the safety person about it when I had to do three trailers in two weeks and flat-spotted two sets of trailer tires.

Zipperhead93

you could also check the treadle valve by hooking up two air gauges, one at the truck brakes and the other at the gladhands, if they read different then that is the problem and if they are the same then ?
check to see if the brake chambers are the same size-if the trailer has larger chambers then there is more braking being applied there.
or are the brake pads on the trailer more aggresive than the ones on the truck.
or are the slack adjusters set at the same length on both the truck and trailer.
just a few things to check

307Kenworth

I see you posted this in October, right around when the weather was getting crappy. We did a lot of transports all fall with our trailer and we went to move either a cat or a forklift this winter and we fought our brakes pulling it out of the parking space, and had our ABS light come on intermittantley. We pulled it in our shop kept it warm and thawed it out for a few days then crawled under it and greased it. The grease pushed out a ton of water and road grime. This week I moved a D5 and a 580 and no sign of the ABS light, brakes came free easily when I hooked on to the trailer, and braking on the highway was much smoother in general.