Brake Pulsation

[DGiles]

I am a new member. I have a 1992 W250 LB and I have sold it. The problem is that it has a brake pulsation that will not quit. I have had the brakes, all of them, replaced and the rotors resurfaced and the pulsation still occurs. Mechanic that did the front brakes has no clue. The pedal is also spongy. It firms up with one pump so I think it may be the flex lines as some in the forum have experienced. Pulsation is really bad when braking hard from say 50 mph. Any suggestions.



Past experience would indicate rotors warped or drums out of round.



Thanks

Don

 

[Greenleaf]

It's not clear if you turned the NEW rotors! That's important as just when you think you have the new rotors on square to the hub... . they are NOT. You'd be surpised to find how much.



The master culinder also sounds like it's suffering some fatigue (?) My front axle flex lines had balloned. I replaced them due to that, not because they flexed too much upon application.



GL

 

[paccool]

I got the rotors on my truck REAL hot due to a problem with the master cylinder. Got the problem fixed and had to replace the pads. Had the rotors turned at that time. When using the brakes easy, they would be fine. If they got just a little warm, the rotors would go back out of round a little and I would get pulsing. I replace the rotors and they were fine after that.



Seems that once the rotors go warped, they would still be out of round when the temps came up, even if they were turned.



I would vote for you to put new rotors on if you didn't before.

 

[DGiles]

Rotors, etc.

Thanks for the replys.



I have had it back to the fellow who installed the front pads and turned the old rotors. He thinks they are true.



I think the warming up situation might have some merit since this thing is fine most of the time. The rotors may warp when hot and thus the pulsation.



Others have suggested that the anti-lock proportional valve may be the cause of the spongy brake pedal. It seems that the anti-lock valve has been a source of trouble along with the improper bleeding techniques.



The pedal is hard as a rock until the power booster comes into play and then it has to be pumped once to get a firm pedal. I think that may be why some owners have suspected the power booster to be the problem.



I'll keep looking for this problem next week.

 

[B.G. Smith]

On some vehicles, improper tightening of the lug nuts can distort the hub/rotor enough to cause pulsation. If the rear shoes were replaced, I bet the soft pedal is due to the fact that the new shoes don't exactly fit the drums and what you are feeling is the pressure being applied causing the shoes to conform to the drums. Years ago, a good brake shop would "arc" the brake shoes (grind them to fit the added diameter of the worn drums). Try applying the E-brake then start the engine and see what the pedal feels like. bg

 

[Paychk]

Others have suggested that the anti-lock proportional valve may be the cause of the spongy brake pedal. It seems that the anti-lock valve has been a source of trouble along with the improper bleeding techniques.

One way to test the RWAL valve is to disconnect the brake line for the rear wheels @ the master cylinder and plug the port. Then test the brakes; if the sponginess is gone the accumulator valve is hanging open and the first use of the brake is filling it.



PS Don't test the truck on public roads with the rear brakes disconnected.

 

[dan brooks]

Don't rule out the rear brakes as causing the pulsation. I fought bad problems on my old Power Wagon that turned out to be an out-of-round drum, and it would shake the whole truck bad enough it felt like it was going to self destruct. The CTD also gets some shaking at times, and it seems to settle down if I adjust the rear brakes.

 

[ARhine]

Don't rule out the rear brakes as causing the pulsation. I fought bad problems on my old Power Wagon that turned out to be an out-of-round drum, and it would shake the whole truck bad enough it felt like it was going to self destruct. The CTD also gets some shaking at times, and it seems to settle down if I adjust the rear brakes.

Second that. We fought the problem on a 95 gasser dodge my dad had. Ended up being bad rear drum.

 

[DieselBuzz]

My guess: the back shoes

I've found that if my pedal is low and comes up with one pump that my back shoes need adjusting. I second BG Smith's idea of testing them with the E-brake set, seems a quick way to check before you take anything apart.

 

[BHiggins]

I had the RWAL valve go bad on my 1990 first gen, it felt EXACTLY like a bad master cylinder, so much so that I replaced the master twice thinking I got defective units. Finally, I plumbed around the anti-lock valve and did away with it. I CANNOT recommend you do this to your truck, because I don't think anti-lock was an option and altering your factory brakes can come back to haunt you... but it worked for me.

 

[DGiles]

More on Brake pulsation.

I replaced the RWAL and carefully bleed the brakes. Also set up my dial indicator and measured the run-out on the left front rotor and found it had . 016" run-out. I took it to another shop and they turned the rotors and the problem is gone. The owner of the shop said it looked like the original shop had not turned the rotors at all.



Truck was picked up by the new owner and is on the way to Alaska now. Sorry to see it go after 13 years but I suppose it was time.



Thanks for all of the assistance. It looks like I will be moving into the 2nd generation forum now since I bought a 1999 CTD 3500 QC. It is a flatbed and more truck than I need but is in great shape so I could not turn it down.



Don

 

[JLEONARD]

My son owns an auto repair shop and turns EVERY new rotor he installs. He has never gotten a return because of pulsating brakes.

While it's possible that new rotors can be true, turning them insures it.

Jay