new brakes + dual piston caliper + seized piston = trouble

[Winston]

Labman, isn't it often tough to polish out the corrosion in the caliper cylinder?

Also, doesnt the corrosion happen where the fluid is? I mean doesn't old fluid contribute to corrosion? I am interested in your opinion due to the many rebuilds you have done.

I always thought frequent fluid changes helped prevent internal corrosion of the brake system componenents. Also, I always found it hard to believe that the dark brown fluid that comes out has similar properties as the original fluid. I would love to see a uoa on that brown gunky fluid.

 

[labman]

I don't think the cylinder wall is very important. We always cleaned them up with a round wire brush in an air drill, but put some badly pitted ones back in service. Nothing rides in the cylinder bore, there is a small gap between the piston and bore. As long as the piston wall is smooth and the seal OK, it slides freely and doesn't leak. I can remember several sets of ghastly old 4 piston Corvette calipers that worked fine with new pistons and seals. I never remember discarding a caliper casting. They were always an acceptable core.

As I have said, the big problem area is the side of the piston between the seal and the boot. This is more so later in the life of the pads and the boot. Maybe also more so, here in the rust belt with salt around the edge of the boot just when it is the coldest and least resilient. The part of the piston originally moving in and out of the seal having its plating abraded, slowly moves out to where it is exposed, not to the fluid, but whatever of the hard, cruel, corrosive world the boot lets by. Eventually rust builds up on it, and causes problems when pushed back through the seal at pad change, sticking or leaking.

There has been a link posted a couple of times here showing pictures of corroded pistons. The text discussed the build up of copper in the fluid saying it becomes corrosive when the copper becomes too high and starts to corrode the steel. I am sure old brown fluid is high in iron, maybe copper too, and moisture. However, in rebuilding/replacing the caliper, enough fluid is discarded that it is all replaced before it becomes a problem.

The exception may be the Bosch ABS. I don't have much experience with it. The manufacturers do seem to be recommending 2 year flushes with it. There are a lot of reports of problems with it. I hope if the ABS on my Cavalier was Bosch, it would say so on it. I haven't been able to find any identification on it. The brakes have gone 4 years and 65K without anything more than looking at the outside of the fluid reservoir. I seldom hear of problems with GM and Ford ABS.

 

[JHZR2]

bosch abs in BMWs reccomend 1 year flushes... however, I believe 1 year dot4 flushes were reccomended before the cars had ABS.

GM reccomends in not so many words to link the vehicle up to the tech II machine to cycle the solenoids and flush the fluid. Ford and all the rest have a similar machine.

 

[Willy_G]

So, did you get new calipers?

 

[JHZR2]

yep, did both since the van does have 196k on it... everything works liek a charm now. The rotor that was hot didnt show any signs of discoloration or whatnot, and the pad was A-OK, so we reused it.

Thanks all!