Brakes are grinding - not pads
 

1978 300D
I'm getting grinding sounds when I brake at slow speeds. I haven't done anything to the brake system, and I've checked the pads and they're good.

I've read stuff on here about sanding down the rotors and/or the pads. I guess it comes from a smoothness of the two? Any thoughts on what the griding is and how I can fix it?

It could be a bad wheel bearing. They will sometimes make a grinding sound during brake application.

it also could be the rear e-brake shoes dragging. be sure and pull the rear rotors and inspect the e-brake shoes.

I have the same thing, I'll check bearings and probably have the rotors turned. Sometimes after some hard braking the rotors sill get hard spots and that can make a noise, as well as eat grooves in your pads.

Interesting. I have the same issue with my 126 - Just replaced everything brake related and I have a similar grinding sound. Are there any other symptoms of bad wheel bearings?

It was the left rear inside pad. So I need to replace it. I got it out without moving the piston - how do you move the piston back?

To move the piston in, use a vice grips or channel locks and the old pad, grip against the caliper, and squeeze the pad in toward the caliper. It may make it easier to pop the cap on your break fluid reservoir (to allow the pressure to get out).

Other people have suggested using a thick heavy screwdriver and using it as a lever to push the piston and pad back in, but I have never done it that way, so I don't quite know how that goes.

If it is just one pad worn, either someone did an incomplete brake job (not very likely?), or you may have a piston that is sticking.

Just one pad wearing may indicate a stuck caliper piston. Check bearings, jack up car, and put your hands at the 12 o'clock and 6 o'clock position and alternately push/pull, no play allowed! Then spin to see if you hear anything.

Sure its not a dragging dust shield?

Sounds easy enough.

This may be the case. What should I do about it?

Big thanks

12 & 6 o'clock of what? Also, how do you check bearings?

Think of an old analog clock. 12 is up top 6 is at the bottom.

When my bearings went bad, there was no play, but spinning the wheel felt and sounded like spinning a very quite ratchet wrench, and I could feel the wheel try to stop with each "click".

So I don't have to stay up 'til midnight to check my bearings?

The tire.

well, I appreciate the help folks.

The problem looks to be a stuck piston causing the rear inside pad to wear down which caused the grinding.

I've got to figure out how to fix the piston. My bad, the title should be changed as it is the pads.

Any tips to fix a stuck piston?

replace the caliper.


it'll cost anywhere from $30-80 after core return, usually.

You might be able to fix it with a new seal and boot. You won't know until you get the pistons out and inspect the pistons and bores. If either is scored, pitted or excessively rusted, the caliper should be replaced. If the piston is just "gunked up," a new seal and boot might be all you need.

What ever you do to one side of the axle should be done on the other.

Try a soft rubber mallet and hit the face of the piston or a small hammer against the old pad laying against the face......do not touch or damage the sides of the piston. If it is dislodged it should spin freely in the cylinder clockwise or counterclock wise. Truefully...even if it can be dislodged the scouring from the rust will ruin the seal and cause it to leak....the caliper cylinders should be honed and rebuilt to be safe.

It's a little tough to hone an opposed, dual piston caliper. How do you do it?

Your right.....forgot we were talking about MB rears. The best you could do is clean the cylinders and pistons up with some fine steel wool.....if not replace the whole caliper.

It's cheaper and more conducive to :) if you replace.

Should I replace both sides?

Yep, brakes, tires, and suspention work in pair...

Good luck, it shouldnt be too bad
~Nate

Alright, thanks everybody. I'll go down to good ol' fps autoparts here in Sacramento and pick up two calipers, rear pads and maybe some grease and then the fun begins.

Keep us posted.

well, took awhile to get the Cardone ATEs calipers. I don't have time to buy them from Mercedes shop although the price looked fair. I paid $89 after calling around - I was quoted 149 from Carquest! I know I could have gotten them cheaper online, but I need to get it road ready asap. I've gotten one caliper off and took it in for CORE return.

Question: On the calipers there are two holes. One for the hose - and what's the other one? On my old caliper it was closed off with a small device. On this one it has a silver screw with an opening on the top. Shouldn't they have included something to cover it up, or does it let air out or something?

The second hole is for the bleeder valve. Did you already exchange your old calipers?

So should it have a seal on it when I'm not bleeding or does it work some other way? I haven't exchanged them yet - haven't really started yet, although I did try to get the brake hose out, but had some trouble because I was twisting the brake line above it/connected to it. Anyone know the best method of taking the brake hoses off?

unless your bleed valve is perfect you should consider buying a new one.

Tom W

The best method of removing the brake hoses is to remove the hose from the brake line first. Then, the entire caliper and hose is free of the vehicle so that you can easily remove the hose from the caliper.........on a workbench.

But, this method is fraught with risk for a new mechanic who has not previously cracked that brake hose from the brake line. There is significant risk of rounding the nut and/or twisting the brake line.

So, I might suggest that you attempt to loosen the hose from the caliper while the brake hose remains connected to the brake line. It's not the most pleasant task, but it can be done. The biggest problem with this approach is the fact that you cannot use a box wrench and must suffer with an open end wrench. It's possible that the result will be a rounded brake hose...........however, if you apply a bit of heat to the caliper with a propane torch, you'll free up the threads so that you have a chance of success with the open end wrench. Try not to force the open end wrench so hard that you round the flats on the brake hose. Also, sometimes a sharp rap with a hammer will have the desired effect.........when greater and greater torque just rounds the flats.

If you start to round the brake hose, post again and we can talk you through the procedure to remove the brake hose from the brake line.

If you have already broken the brake hose fitting loose at the caliper, remove the caliper and twist it off the hose instead of trying to unscrew the hose with the caliper still installed.

If your brake hoses are original, this would be the perfect opportunity to renew them. That said, take great care if you separate the hose from the hard line. Mess that up and you will complicate matters greatly.

Since you are replacing the calipers, the hose may not "index" correctly on the new caliper anyway, which would necessitate loosening the upper end of the hose to avoid a twist in the hose.

That's a good point, TF. If the threads don't seat at the proper place, the line will twist and you might as well have taken it off the hard line to begin with.

The removal from the hardline is fraught with risk..........as you know.........probably should not be attempted without proper metric flare nut wrench.

Ok, so it's a bleed valve. Well, it came with the caliper, but it doesn't have a top (covering the valve like my old one) to it. Do I take it out and put something else in, cover it up, or put the old valve (with a cover) in it?

It seemed to make more sense to take the caliper off first and then unscrew the hose rather than twisting it up. So, the caliper is off and the hose hangs from the brake line (hardline?) How do I get the darn thing off of the hardline - it seems that with every turn the hardline turns too.

The nut on the hard line should turn independently of the hard line itself. Your nut might be frozen to the line.

Have you considered using just the old cap on the new valve? The cap is just a dust cover, which is not to say that it's not important.

Do not let the hardline turn. You need two wrenches..........one of which (the smaller one) would be better as a flare nut wrench. The smaller nut turns inside the brake hose. The danger is rounding the corners on the smaller nut. If you do that.........you're toast.

Get a bit of heat on the outside of the brake hose (carefully) in an attempt to break the bonds of the nut.

You're working in a difficult area.........two wrenches and some patience are mandatory.

OK sounds good. Thanks!

That's what I was thinking - I turned in one old one with the old caliper for the CORE return. I'll try calling them to get it back.

That's the "exchange" that I was talking about.

I never turn in a core until the job is complete.

Changed both rear calipers, changed the brake hoses, inserted the new pads, bled and changed the brake fluid.

While bleeding the front brakes I noticed that the pads were rubbing the disk. Took it out for a drive and there was a sound like the pad was rubbing the disk at one point every rotation. It is a new sound that I didn't have before I did work on the rear brakes.

hmm, it could just be a little rust on the rotor from sitting for a while, or you could have a warped rotor. where did you notice the pad hitting? was it on the ridge around the rotor? did youmeasure your rotor thickness up front? it may be below the limit.

Haven't heard that sound after driving awhile. All is good. Thanks for all your useful advice everyone!

See if you can get stainless pistons for the calipers, otherwise it may be less expensive to get a caliper from a junk yard and just swap it. If you rebuild one caliper make sure you the the other one on the same "axle".
Mike