Hot Brakes - PeachParts Mercedes-Benz Forum

kweimer · #1 08-07-2002, 01:54 PM

HELP! I just replaced pads and brake hoses on rear brakes. Since I could not get the master cylinder to push fluid to the rear calipers, I replaced master cylinder with "good" used master cylinder. Bled brakes no problem. Out driving today and brakes "dragging"----got home and both wheels "red hot" (felt that way anyway). Front OK. Any ideas what my problem is? I am at a total loss (I know bad swollen hoses can cause this problem---but hoses new OEM).

JimSmith · #2 08-07-2002, 07:07 PM

K.Weimer,

I would guess your caliper bore/piston seals are shot. When you apply the brakes, this seal, a rectangular cross section "O"-ring, gets deformed by the piston moving slightly. The materials, surface finishes and dimensions are all set so that this ring pulls the piston, thus the pad, back from the disc ever so slightly when you take your foot off the pedal. This is the only feature in the brake system to accomplish this task, as there are no springs and lifting the brake pedal does not cause a suction in the brake lines to pull the piston back. Once this seal gets worn, it may still seal, but it will slide on the piston surface rather than grab it, and then there is nothing to pull the piston back. So your brakes grab. The fix is to replace the seal, but if you take the brakes apart to do this and find scratches or corrosion on the bore or piston, you need new calipers. It is beyond the capability of the DIY'er to refinish the bore or the piston, as any material removed will alter the basic design set up of clearances and likely surface finish, and the seal will not pull the piston back. Or worse, it may leak and you will have very poor braking performance.

Hope this helps, Jim

240Joe · #3 08-07-2002, 07:45 PM

My 300D would intermittently heat up the left rear brake. I replaced the caliper and the problem was solved. I found a rebuild at a good price with one day turnaround at advance auto.

Since you have both back brakes hanging up, you would think the master cylinder is bad, but since you changed that out, it has to be the calipers. You should be able to tell by trying to push the pistons back into the bores on both sides. Mine would not move on one side.

By the way, did the car sit for awhile? I believe that's what caused mine to freeze up. PO didn't drive it much.

Joe

kweimer · #4 08-07-2002, 08:16 PM

Thanks for the ideas. I will pick up seal kit for the calipers and start there. If the bores/pistons have any of the described faults I will place seal kits "on the shelf" for next time, and get rebuilt calipers. Think I'll do the front calipers too---all four are old---very old. I have my work cut out for me tomorrow.

P.E.Haiges · #5 08-08-2002, 09:08 AM

The seals are now against a section of the caliper bore that they were not touching before because of the new, thicker pads. This section of the caliper bore may be corroded and causes to seals to not work properly. I would first inspect the bores and if they look OK, try a rebulid.

AS far as the front ones are concerned, as long as they work OK and don't leak, I wouldn't touch them. Mine were "old" 12 years ago and the still work fine and don't leak.

If it ain't broke, don't fix it.

P E H

kweimer · #6 08-08-2002, 10:19 AM

I too subscribe to the "if it ain't broke don't fix it" line of thought. The reason I thought I would go ahead and rework the front calipers is because the front wheels get a little hotter than I like too---compared to my other two "daily drivers"---1975 300D's. Over the 40 years I have driven Mercedes, I have had very little caliper trouble---which is why this problem kind of threw me---having two calipers freeze up at the same time. Thanks for the tips---I'm still learning.

autozen · #7 08-08-2002, 10:51 AM

Since you didn't state it, I am assuming you did not have this problem before you installed the new pads and hoses. If the calipers are creating the problem, because you moved the pistons to a new position, I'd say chances of both calipers developing the exact same symptoms are the same as winning the lottery. The same odds apply to two new hoses. I'm betting you installed a bad "good" used master cylinder or one from a rear drum brake car with a residual pressure valve. After 30 years of pushing caliper pistons back, as a professional Mercedes Benz mechanic I'm pretty sure that's your problem.

Good luck,
Peter

P.E.Haiges · #8 08-08-2002, 06:36 PM

Autozen,

I had 2 front calipers on my '79 300SD go bad at the same time. They started leaking when it got cold outside. When the weather warmed up, they stopped leaking but started leaking again with another cold spell. Rebuild kit fixed both and they are working perfectly years later. So I guess it is possible to win the lottery.

P E H

kweimer · #9 08-08-2002, 08:58 PM

Peter;
My original problem was the left rear wheel getting "red hot" to the extent that the left rear caliper was unrepairable. At that point, I replaced the left rear caliper with a "good used" caliper and replaced pads and hoses on both rear wheels. It is after this procedure that both rear calipers got hot to the point of virtually locking up. The "good used" master cylinder came off a '75 240D same as the '75 240D I am working on; however, your point on "good used " master cylinder is well taken. Along with caliper rebuild kits and hoses for front and rear, I have also purchased a new master cylinder. As of right now, I have just finished rebuilding the left rear caliper and installed the new master cylinder. Tomorrow I will rebuild the right rear caliper and both front calipers along with replacing both front hoses. You would not believe---well maybe you would---the crud that came into my "bubble jar" while bleeding the brakes. Piston and inner bore of rebuilt caliper was in excellent condition after minor cleaning with #600 grit sandpaper. I hope to finish this job tomorrow---with some degree of anxiety. Thanks, folks, for these words of advice that keep coming in.

autozen · #10 08-09-2002, 11:47 AM

K. Weimer,

You left out an important detail about the feft caliper. It could have been a caliper problem, but it's just as well you're doing the M/C. You will be good to go with a whole new sytem. Don't forget to check for and clean out any brake fluid in the booster. Good luck.

PEH,

Don't buy too many tickets. You are referring to a natural occurrence when components have been in service the same amount of time. Whenever I had a client come in with a leaking caliper I recommended doing both, because the other one was probably not far behind. On a few occasions the client asked to do the other at a later date for budget reasons. Sure enough, within a month or two the other would start leaking. I was saying that in K.Weimers case(before the new info)the odds are very great that two previously functioning calipers would both fail exactly the same when the pistons were pushed back.

Peter

kweimer · #11 08-09-2002, 01:54 PM

Peter;
I am doing the whole brake system---just the amount of crud and deposits on the caliper bores and pistons that I have found warrants a complete system rebuild---although the M/C is new OEM. I checked the booster for fluid when I replaced the M/C---bone dry.

Fisherman · #12 08-09-2002, 07:07 PM

I just looked at your list of cars. Are you a used car dealership or just some kind of sick MBZ Diesel nut...

Probably the latter which is okay by me...

engatwork · #13 08-09-2002, 08:10 PM

Fisherman - I have met Ken - that is an active list. He is good people and a diesel nut like the rest of us and obviously MB diesels is his hobby

.

kweimer · #14 08-09-2002, 09:32 PM

Fisherman;
When it comes to diesels, I'm a real sick'o nut---I even pick up a few gasoline models---but its the diesels that really have me hooked. My collection has even attracted the attention of our local sheriff's department---all ended well; told me to give them a call if they could help.

kweimer · #15 08-09-2002, 10:36 PM

Many thanks to Jim, Joe, PEH and Peter for your input. Brakes are fixed---after lengthly test drive, 240D stops in perfect straight line and no hot wheels.