Periodic front brake drag problem - PeachParts Mercedes-Benz Forum

swheele2 · #1 07-16-2009, 08:31 PM

My brakes work fine most of the time, but periodically the front will start dragging significantly (car feels very sluggish, engine temp climbs to about 100 C). Eventually the problem will "fix itself" and it will start driving normally again (this happened a few weeks ago 50 miles into a 100 mile drive. After a few miles of dragging brakes they loosened back up and worked fine the rest of the drive).

I installed new (rebuilt, ATE) calipers and new flexible brake hoses. The brake fluid has been flushed multiple times at this point (I also replaced the rear calipers) and the fluid is perfectly clean/clear.

Since both front wheels are dragging equally, I assume it's not a problem with a rebuilt caliper. The previous owner had a new master cylinder installed May, 2008, so the MC should be fine, but I'm not sure what else could be the problem. Could it be that the "new" MC is bad?

Any suggestions? All of the previous discussions of brake dragging I can find are not intermittent like this (or at least that wasn't the impression I got).

1984 300D Turbo (200K miles)

Steven

Matt L · #2 07-16-2009, 08:50 PM

Can you adjust the pushrod on that MC? If the cylinder isn't bad, it could be the pushrod adjustment.

swheele2 · #3 07-16-2009, 08:56 PM

I do not know if it can be adjusted or not. Anyone know?

t walgamuth · #4 07-17-2009, 05:27 AM

I believe the bolt that attaches it to the brake lever is an eccentric and can be adjusted some. Just loosten the nut and turn the bolt until you have it where you want it and tighten.

tbomachines · #5 07-17-2009, 08:17 AM

You must be going through the pads like crazy, no?

swheele2 · #6 07-17-2009, 10:11 AM

I did burn through a set of front pads rather quickly.

I'll take a look at adjusting the push rod tonight. Thanks for the suggestion.

swheele2 · #7 07-18-2009, 08:29 AM

I can't find any adjustment on the pushrod. The only adjustment at the pedal end is for the brake light switch. It's already set to allow maximum upward pedal travel, so I don't think it's a case of the brake light switch hindering the pedal movement.

swheele2 · #8 07-29-2009, 09:48 AM

I'm still stuck on this one. I went ahead and swapped out the MC for a new one. Same problem.

The original poster in this thread from 2006 seems to have had similar difficulties (W123 Dragging brake question). In the end he just drilled into the MC to simulate a shorted push-rod. I am unwilling to do that.

In my car the pushrod attaches to the brake pedal via a "pin". There is no eccentric bolt. Is there some sort of adjustment inside the booster? That would seem like a very odd design, but I can't find any way to adjust the pushrod at the ends.

Anyone have any thoughts?

tbomachines · #9 07-29-2009, 09:57 AM

Quote:

Originally Posted by swheele2

I'm still stuck on this one. I went ahead and swapped out the MC for a new one. Same problem.

The original poster in this thread from 2006 seems to have had similar difficulties (W123 Dragging brake question). In the end he just drilled into the MC to simulate a shorted push-rod. I am unwilling to do that.

In my car the pushrod attaches to the brake pedal via a "pin". There is no eccentric bolt. Is there some sort of adjustment inside the booster? That would seem like a very odd design, but I can't find any way to adjust the pushrod at the ends.

Anyone have any thoughts?

Did you toss your old MC? Maybe you could drill into that one and see if it helps. Not sure I'd want to drive a car with a modified MC for very long though.

swheele2 · #10 07-29-2009, 04:53 PM

Quote:

Originally Posted by tbomachines

Did you toss your old MC? Maybe you could drill into that one and see if it helps. Not sure I'd want to drive a car with a modified MC for very long though.

I kept the old MC (which is only about a year old and probably perfectly fine). I'm not really comfortable driving on a modified MC though. I'd rather drive with periodically dragging brakes.

Chas H · #11 07-29-2009, 05:42 PM

The pushrod might be adjustable inside the booster after the M/C is removed. But don't think that's your problem. Since the PO replaced the M/C there is a chance the booster has been damaged by brake fluid when the old M/C failed. What you have is a sticky booster.

kingdoc1 · #12 07-29-2009, 06:38 PM

The fact that the brakes don't stick all the time would rule out the pushrod length, since it is fixed and would be in the same position every time you apply the brakes.

The master cylinder piston could be sticking, or as suggested earlier, the booster.

I am not sure if the car uses a proportioning valve, but if so it is another possibility.

barry123400 · #13 07-29-2009, 09:18 PM

No external valve past the master cylinder. I would almost think the booster is all that is left in the realm of possibilities yet it is strange.

You can send me your one year old master cylinder for further testing though.

swheele2 · #14 07-30-2009, 09:57 AM

Quote:

Originally Posted by barry123400

No external valve past the master cylinder. I would almost think the booster is all that is left in the realm of possibilities yet it is strange.

You can send me your one year old master cylinder for further testing though.

That is a good point about the pushrod being unlikely, since the problem only happens periodically. I agree that the booster is the only real option left. Every other brake component has been checked/replaced.

I'm traveling the next couple of weeks but will put in a new booster when I return. I'll let you know how it goes.

swheele2 · #15 08-15-2009, 05:33 PM

Quote:

Originally Posted by Chas H

The pushrod might be adjustable inside the booster after the M/C is removed. But don't think that's your problem. Since the PO replaced the M/C there is a chance the booster has been damaged by brake fluid when the old M/C failed. What you have is a sticky booster.

It was a sticky booster after all (or at least that appears to have been the problem). I finally had a chance to replace the booster and when I pulled off the recently installed master cylinder the booster probably had a pint of brake fluid in it. I'm a little embarrassed that I didn't notice that when I put in the new MC a few weeks ago. Regardless, the new booster seems to have fixed the problem, but since the severe brake dragging was not a constant problem I'll have to drive it around for a few weeks to be sure. Looks good so far though.

Thanks to everyone for their help.