frozen rear brake calipers

Hello,
I did the brakes on all four wheels of my '89 300e a year or two ago and discovered today when I had the tires rotated that both rear brake calipers
are frozen and have not been working--probably since I bought the car about 20k ago. I only drive it once or twice a week so they don't get much of a work-out. I hate to replace the calipers since they are kind of expensive and obviously not leaking (yet). Does anyone have any tips on what it takes to free up the pistons? I'm thinking WD-40 and some rigorous brake pedal exercise but am ready to dis-assemble the calipers and take some emery cloth to the cylinders if need be.
Thanks,
Mike
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DO NOT USE WD40!!! It will swell up the caliper seal and cause bigger problem. If you are handy, you can remove pad on one side and let the piston move toward the rotor and then push back... exercising the piston.
Otherwise, have someone rebuild it. There are kits to do this job. Rockauto has the rear calipers for $55 rebuilt.
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If they are frozen, just trying to get them to move is useless. They froze for a reason, and that is that the pistons and or cylinder are corroded, usually from water getting past the boots. And once they get to that point, rebuilding them yourself is out.
Also, if you did a brake job a year or two ago on all wheels, I don't see how these calipers could have been frozen since you bought the car. In cars I've had, when calipers seize, it's usually obvious, as you can smell buring brake pads, feel uneven braking, etc.
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When I did the brakes all I replaced was the pads and rotors but did nothing to the calipers except push them back with a c-clamp to get the new shoes in. As I recall they did not retract easily and I suspect now that they were frozen then and have remained so until now. I pulled them off this morning and dowsed them with a silicon lubricant while attempting to work them in and out with a c-clamp and brake peddle pressure and although it appears I loosened up both pistons on the driver's side the passenger side will not budge so I went ahead and ordered a pair of rebuilt calipers from RockAuto. Thank you all for your help and comments. Mike

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I once made the mistake of compressing calipers without loosening the cap on the master cylinder and they froze.
I was able to unstick them afterward, but they failed (leaked) a year or so later.
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I'd take them apart, clean and re-assemble with new rubber. Should be fine. Do not use brake fluid for an assembly lubricant or else they will rust up quickly. Use a grease made for this or silicone grease.

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Assembly lubricant to be used where? The pistons and piston seals should only have brake fluid on them. The high temp grease made for brakes is used on some very select points where the pads/calipers float, if called for by the manufacturers instructions. It's there mainly to try to avoid brake noise.
In my experience, if a caliper has actually seized, it's pointless to think that you can rebuild it by just freeing it up and putting in new seals. They seize because the cylinder/piston are corroded and at that point, the easiest and surest solution is a rebuilt or new caliper.

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I am talking about lubricating the rubber seal for the piston to ease assembly and ensure free movement when working. This is exactly where you do not want to use brake fluid. The reason for this is that brake fluid attracts water as a part of its design. If you use brake fluid as an assembly lubricant it will attract water and cause rust around the piston which will cause it to seize. There is grease made for this. I believe that the w123 Mercedes manual mentions it. I have used silicone grease for decades and have not had any problems. When I was a kid I used brake fluid for assembly and would have to redo the job a couple of years later because it would rust up.
If you can pop the piston(s) with compressed air and the chrome plating cleans up well there is no reason to not continue and clean the rust from the cast iron and re-assemble with new rubber. Pay attention to make sure that the seal groove is spotless or you could have a leak. Sure it is a lot of work but cheaper than new parts.

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I'd like to see a reference for this. Every repair manual I've seen has specifically warned NOT to get any lubricant other than brake fluid anywhere into the system, including the pistons, seals, etc. It would seem pointless as well, because upon assembly, brake fluid is going to be in contact with the seal anyway. It has to be, as that is what the seal is there for, to keep the brake fluid in.

And how is that possible with a FROZEN caliper? When the caliper seizes, the enormous hydraulic pressure can't move the piston, because the piston is frozen in place, from corrosion. There is no way compressed air is going to move it. That works with a piston/caliper that is in good shape, not a seized one.
Pay attention to make sure

It's not that it's a lot of work. It's that the piston/caliper interface needs to be perfect and with a seized one, it's nearly impossible to get it right because corrosion has damaged the piston/ caliper interface area. Which is why a rebuilt or new one IMO, is the best option.
I do know guys who advocate proactively rebuilding calipers, BEFORE they seize. At that point, if you change the piston seals and boot you can do it before the corrosion has caused damage.

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Here is one reference. The W123 manual says the same thing. They say to use "ATE brake cylinder paste". http://mymercedesdealer.com/Benz-Mercedes-Nassau/Benz-Mercedes-Nassau.php
"Permatex Ultra Disc Brake Caliper Lube" says it is for all moving parts including pistons but I have not used it.
Sure brake fluid makes contact with the seal. The difference is that it is on the inside. Not the outside where it will make contact with the moisture in the air. Moisture will get past the dust seal due to permeation and vapor pressure.
It is true that you should not let any petroleum based lubricants contact brake parts. You'll noticed that I said special grease or silicone grease. These will not harm the rubber. Used properly, there will only be a trace amount on the inside of the system.
I think that frozen is too strong of a word. Binding is a better word. What happens is that the rust will prevent the piston from being moved back by the seal causing the brake pad to drag on the rotor. If it were frozen in the forward position you would not be able to drive the car. Using 100 PSI on a 2.5 diameter piston will give about 500 pounds of force on the piston. This is enough to remove it. I suppose that it is possible that it won't, but I have never seen it. You may need to move the piston in with a c-clamp and spray a penetrating oil to help free it up.
As far as using a rebuilt part, how do you think they deal with the rust? The removal of rust will not cause enough of a dimensional change to cause any problems. As long as the chrome plating is not damaged it will work fine.
I have seen manuals that say to assemble with brake fluid and I have seen the problems that this causes.

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Thanks for the link. I did a bit of googling as well and found some others folks also talking about using the brake assemby paste on pistons/calipers. As long as the stuff is made to be compatible, then I agree, it sounds like a good idea. It certainly can't hurt to have the piston surface between the caliper seal and boot coated with a water resistant paste as I would think it would help avoid corrosion if any water gets behind the boot.
I may try it myself in the future. Thanks for the tip.

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You are welcome.

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Doesn't MB recommend flushing the brake fluid on a yearly basis?
Chris

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IIRC, it is every two years, preferably in the spring.
chrismhaney wrote:

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Yes, every two years... which majority of us never do.
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