Hi all!

Have a question and need some help. I have a 1992 w250 that has 227000 miles on it. VERY well maintained.

whenever I put any pressure on the brakes to slow down or come to a stop, the truck hops. HOPS down the road. There is NO pulsing in the brake pedal and there is no pulsing or weird feeling in the steering wheel. The tires are not out of balance as it goes down the road straight and smooth. The shop doesn't know what is causing this, they've had it apart and are inspecting it again.

Any ideas?! Anything is appreciated!!!!

 

RWAL valve sticking.

 

What is that valve and what does it do exactly? Never have had this problem before with a truck.

 

Rear Wheel Anti Lock valve is in the inside of the frame near the rear wheels on the driver's side. It is supposed to modulate the rear wheel brakes to prevent lock up. Sometimes they goof up.

 

As it is garbage. I completely removed and bypassed mine after hours of scanning threads on the REAL system that or trucks have.

 

How do you bypass this valve and I'm assuming that you lose the abs in the rear at that point correct?

 

is it a sensor or an actual valve? All I'm finding in parts stores is sensors. Not sure if I want to bypass it or attempt to fix it

 

i just found a remanufactured hydraulic assembly. Is this what I'm looking for?

 

That is it, but you should probably do a little testing before you replace it. I would temporarily bypass it to see if that resolves the issue. Grease or oil on the brake shoes can cause the brakes to grab and maybe produce your symptoms. Check all of the axle mounts too. The function of that valve is to release hydraulic pressure during wheel lockup, so it would not likely cause higher pressure that would lock up the wheels, IMHO of course...Mark

 

Thanks so much for the help everyone, I'm going to pass this along to the shop and see what they can do with it.

 

You've heard of KISS ?

Keep it simple stupid.

Behind the glove box is the abs controller, unplug it. Drive around

If the hopping stops you're on the right trail. If not , pull the drums, and inspect as Mark suggested. Ive experienced this when my rear brake shoes were oil soaked or the drums became mirror polished.

 

I bought my truck when it was five years old. I never liked how that RWAL worked.

 

Mine did the hop routine on initial stop every day till I junked the RWAL system. When working well, it's performance is marginal, especially compared to more advanced antilock systems.

The sensor is on the differential housing. Here's a delete:

 

Mine too resides in the scrap pile. the thing scared the hell out of me too many times.

 

Thanks all for the help, the shop was going to move to this possibility as soon as they were done finishing looking into their theory. They took my front rotors off again and had them looked at much more closely and it turns out they weren't warped but bent somehow. Brakes are as smooth as glass now!!! The fact that they weren't warped is why we weren't feeling it in the pedal.