W126 front brake caliper repair questions

[donbryce]

Last year about this time the mechanic who safety inspected my car mentioned that one of my front caliper pistons was "sticking". I paid no attention at the time and continued to drive the car, which we use for occasional out of town trips and mostly as a second vehicle around town. I figured with this low mileage, the sticking caliper might not cause me any noticable issues, as the braking worked fine.
Well, I'm now getting a distinct shuddering, but not so I can feel it at the pedal, more like the whole suspension is shaking when brakes applied, especially at highway speeds.
So I pulled both front wheels and checked all 4 pistons by retracting them with a C clamp. The right side is OK, with very little wear on the pads, and they're worn evenly on both sides (ATE calipers BTW). The drivers side however has a barely worn pad on the outside and about 1/8" left on the inner pad. The inner piston can be retracted easily with the C clamp though. I noticed a small tear in the rubber sealing ring on this inner piston too. My thoughts are that water has gotten in and caused some rust to build up, making this piston "stick" so it won't retract normally (which is from the centrifugal force of the spinning rotor, right?).
I didn't check the runout on the rotors with a dial indicator, but spinning them manually they didn't look warped, and I think I'd feel it at the pedal if one was, right?
So what I want to do is remove the caliper, clean up the piston, and fix the tear in the rubber. Yeah I know a kit is cheap, but to where I live, it'll still cost me about $40.00 per side, and I'd like to disassemble first anyway to see how bad the damage is, as I might need a new caliper.
CAN I DISASSEMBLE THE CALIPER/PISTON, AND RE-USE THE SEALS IF IT IS FEASIBLE, OR DO I NEED A KIT? In other words, has anyone successfully reused those seals after cleaning up a piston and bore???

[Brian Carlton]

Originally Posted by donbryce

So what I want to do is remove the caliper, clean up the piston, and fix the tear in the rubber. Yeah I know a kit is cheap, but to where I live, it'll still cost me about $40.00 per side, and I'd like to disassemble first anyway to see how bad the damage is, as I might need a new caliper.
CAN I DISASSEMBLE THE CALIPER/PISTON, AND RE-USE THE SEALS IF IT IS FEASIBLE, OR DO I NEED A KIT? In other words, has anyone successfully reused those seals after cleaning up a piston and bore???

First of all, it's the outer piston that is the problem. No pad wear indicates that it's stuck in the bore. You'll have some difficulty to remove such a piston and you'll have to use the hydraulic pressure of the vehicle to do it. Naturally, you'll need to prevent the inner piston from expelling itself at the same time.

No, you cannot reuse the seals after you have dismantled it. You need new seals and a new rubber boot. And, there is no assurance that the piston is not corroded beyond repair. The piston is plated with very high quality chrome, so, usually, you can polish off any corrosion. But, you'll need to be patient.

Either do the job properly with new seals and new boots for both pistons or just replace the pads and drive it the way it is.

There is a school of thought that states that both calipers should be rebuilt at the same time to ensure no pulling, but, I'm sure you don't want to hear that one...............

[donbryce]

Thanks for the reply Brian. While waiting for an answer, I dug deeper into the forum archives and learned how the piston is retracted (apparantly, this is the job of the sealing rings, as they are deformed when the piston is pushed forward and try to recover their shape when the pressure is removed, pulling the piston back as they do so). There is likely something in there that is preventing the return motion and/or the rubber is tired and lost its elastic strength.
So, a kit is on the way. I'm gambling the price of the kit that the caliper can be rebuilt, and I'm hopeful it can. If not, I'll get a rebuilt caliper and use the kit on the other wheel as preventive maintenance.
Interestingly enough, I found a better price overall (to Canada) from ******** AZ than an outlet in Vancouver BC. I'll save at least $10.00 if the border pricks grab the package for taxes (15%), and about $20.00 if they don't.
I'm not understanding the mechanics here very well though. If you are saying the piston with the most wear is the good one, how/why is this shuddering happening? Presumably the lack of pressure from the opposite piston (which we assume is stuck 'out' in the bore) causes uneven pressure against the rotor face. So would this translate into a make/break contact with the rotor instead of a gripping force to slow the rotor by friction, causing the vibration?

[Brian Carlton]

Originally Posted by donbryce

I'm not understanding the mechanics here very well though. If you are saying the piston with the most wear is the good one, how/why is this shuddering happening? Presumably the lack of pressure from the opposite piston (which we assume is stuck 'out' in the bore) causes uneven pressure against the rotor face. So would this translate into a make/break contact with the rotor instead of a gripping force to slow the rotor by friction, causing the vibration?

Cant vouch for the cause of the vibration, but, the fact that one pad is worn all the way and the opposite pad is not worn whatsoever deems that caliper suspect.

It's possible that the vibration is unrelated to the problem with the caliper. But, the caliper is clearly a problem and should be repaired.

I had the same situation on a rear caliper on the '86. Wore one pad down to nothing in 10K miles.........and I hardly use the brakes at all. No vibration associated with this condition.

[t walgamuth]

braking on one side might cause excessive heat on the side doing the work. i suppose it might cause some warpage.

if fixing the calipers doesnt do the trick you might need a new rotor.

tom w