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.
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.
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.
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
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.
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
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.
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.
Here is one reference. The W123 manual says the same thing. They say to use
"ATE brake cylinder paste".
"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
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
I have seen manuals that say to assemble with brake fluid and I have seen
the problems that this causes.
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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