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Good-looking used caliper might be a safe bet...
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Posted by RayF (more from RayF) on Sat, 2 Jul 2005 13:42:25 Share Post by Email
In Reply to: How hard to rebuild caliper, Dylan, Sat, 2 Jul 2005 11:37:04
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...if you can find one not too rusty in a pull-it-yourself junk yard.
Almost no one rebuilds calipers themselves.
Brakes are incredibly robust (besides caring about you, no car maker wants to face your pitbull lawyer in a lawsuit over bad ones) and if they were evidently working on the other car and not nearly choked up with overgrown heavy rust everywhere, the caliper you pull from a junkyard, once bled, is still good for the long haul.
Not being Saab specific, it's usually rear calipers that go bad, in northern climates, from road salt getting in the front part of the piston bore, inside the dust seal but ahead of the o-ring that seals to the piston, and rusting the bore, till it sticks tight to the piston and the piston refuses to return. Rear brakes carry so little of the braking load and they move such tiny distances in use, there's not enough motion action going on there to bust up and work out any developing rust.
And also when rotors get old and thin, especially when they've been turned (Saab doesn't recommend this), then at the end of pads' life, the piston is so far extended in the bore, it cocks sideways and jams so it won't return.
If I wanted to rebuild one I'd first have to make sure I could get the piston out. Do this while it's still in the car and hooked to the hydraulics if it moves at all.
A compressor and an air chisel and air powered cutoff tool with stiff wire brush and goggles are almost necessary tools to start tackling rebuilding a caliper.
If the piston is stuck solid, use an air chisel with a blunt tip tool(NOT a chisel bit), angled in, to make vibration action to get it moving in its bore, then use the pedal to get it out. (If your problem were a stuck caliper, you could stop here, work it in and out a few times, and call it sort of fixed, until it stuck again.) Of course if you mangle it doing this you'll need to get a junkyard caliper or a rebuilt anyway.
Off the car, a bench-mounted master cylinder with pedal attached for leverage could be hooked up to blast pistons out - - but that's a lot of trouble.
The wire brush will clean the rust off the caliper body, down to solid iron, with a little hammering and chisel chipping too. This is mostly cosmetic except at pad-mounting sliding surfaces and right in at the piston bore, but it will make you feel better about your job. Can make a cloud of dust.
I don't believe the quality of the rubber parts you get in rebuild kits ever equals that of the originals.
Getting the piston dust boot out: Usually this has a spring steel ring around its outer edge that holds it into a groove in the caliper. Often heavily rusted in. Dig away with an old screwdriver. Then, the groove needs to be cleaned out well.
Pistons are often stainless steel, or HEAVILY chrome plated low-rusting iron alloy, and any rust on them is probably just adhered to the chrome. They need to be perfectly smooth so be gentle removing it. Scotchbrite Roloc pads on an air cutoff tool; maybe even a soak in the sink and a scrub with a sink pad?
The inside outermost part of the piston bore could have lots of rust. It all needs to be removed, and it really doesn't matter out here if there's pitting of the iron, because you're out past the sealing area.
Then there's the piston seal, a square-shouldered o-ring in a groove in the bore wall. Remove it with a dental pick or similar tool, and clean everything up well.
I don't know how the Saab piston adjusting mechanism on the rear brakes works and it could need attention. Ditto the e-brake mechanisms.
Everything needs to be really clean on reassembly; wash out with alcohol and blow clean and dry; then lube new seals and piston and bore with brake fluid or brake assembly lube before putting it back together.
It's nice to get a new bleeder screw, and run it in and out several times to make sure there's no rust left in the threads. At least free up your existing bleeder screw and make sure it will stay free.


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