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My experience differs. I have found that siezed calipers involved failed or leaking outer dust boots that allow the piston to corrode in the caliper bore. In some cases the caliper rust just creeped under the seal. If the seal is working well water will not get in there. I also feel that salt makes things worse, but directing the hose or pressure sprayer on the brakes may actually make things worse. The rear caliper that I replaced last fall had one seal that seemed to simply rot in one place allowing moisture to get in. I don't know if the outer seals will fail if exposed to oil and or brake fluid. I would like to have one to cut up into measured lengths to put into jars of different materials to observe swelling or rot.
If the seals are working, the fluid in the calipers will not be able to attract any moisture from the air at all. When I bleed my brakes the fluid was quite clean at the calipers. It only gets dark at the reservoir where these stupid european brake systems are vented to atmosphere! Many US designs have a diaphragm at the reservoir that isolates the brake fluid from the atmosphere. I change the fluid at the reservoir every year for this reason.
I have seen some references to the cavity under the caliper outer seals being filled with a silicone grease. I would love to have this but lack required info and materials specs and sources. Anyone???
I had lots of problems in the 70's and 80's with ATE calipers on Volvo's. They would sieze up and with 10 caliper pistons on a vehicle, new seals were difficult and expensive. I got a sample of a brake assembly lubricant and I expect that is was silicone based. I had good success in injecting this materal into the outer seal cavity with a syringe. These injections were able to free up siezed pistons that could be exercised to the point of being quite free. There would have resulted a slush of this lub and corrosion product behind the seals. These pistons with restored freedom worked for years and years with no further problems. I had success with this on my own vehicles and a friends. After this, I injected the fluid throught the seals of every piston as a preventative. The small needle cut (not hole) in the seal was wet with the fluid and did not create any problems. One could work the needle into the piston-caliper interface at the 12 o'clock position so that the fluid would wick down throught the complete annular interface. Sadly, I left the remained if this fluid when I moved to Kansas.
And as I have stated before, if you are trying to replace the fluid in the calipers, a basic bleed operation is probably not going to work. If you jack up all four corners and remove the wheels, push the pistons back in with the bleeders open. Close the bleeders and pump them out. Repeat. This will work stagnet fluid out of the caliper cavities that I believe that a basic bleed will not manage effectively. Don't push the petal to the floor or the master cylinder seals may be damaged. Limit travel with a block of wood or something.
Some times a caliper will sieze solid. This will result in that side of the rotor not getting pressure and that surface will stop wearing and will get rusty. A stiff piston that moves under high forces only will advance on the rotor but will not retract much at all. This will result in brake drag and uneven wear. Often the second case advances to become the first case.
posted by 207.43.19...
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