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Eric_Shea Eric_Shea is offline
PMB Performance
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Join Date: Dec 2000
Location: Sandy, UT USA
Posts: 1,065
Originally Posted by doublebuffle View Post
I elected not to try the grease gun trick because of the "clean up" required. Great method though.

I'm shocked that none has mentioned this already:

In all likelihood, if you were able to remove your pads, the pistons are not all that stuck. You must have moved the piston back to relieve pressure from the pad!

With a c-clamp, compress the piston slightly into the bore. This will 'free' the piston and then you can shoot it out with a tiny bit of air pressure. I used 90 PSI, but just make sure you have a piece of wood and a ton of rags to catch the thing. If it shoots across the garage it will be damaged (and you will have a new project).

If this does not work, reinstall on the car and use brake pressure to move it, (substitute one brake pad with something thinner).
If your car has been sitting for any measurable amount of time, air will not work. Moving the piston helps and it can increase your odds but, fluid is really the only way. Air really only works on an operational caliper.

For most DIY guys and gals out there, leaving them on the car may not be the best solution. It is "extremely" messy and, once you've popped a piston you now have an open system. You'll need plenty of M10x1 plugs to cap off the master cylinder as you move along. You'll also need to plug the open bore which isn't easy on the car. I still like the grease gun as you can remove the tip and most have a 10x1 fitting there to screw into the calipers supply inlet. You can do this more conveniently on the bench and as a fluid, it's rather safe. The piston should "plop" out in your hand vs. air which can come out at bullet velocity. This not only damages fingers but piston tops as well. I've seen countless pistons chipped this way or... WORSE, with plier marks all over them. NO!!! Air can also damage the knockback pin in the opposite bore (if so equipped), bending it and causing your freshly rebuilt caliper to bind.

Brake systems are one of those areas on your car that absolutely hate sitting. The plating on your caliper bore wears off over time and, the fluid is hygroscopic (it attracts water). The water gets in your system and likes to gather around the rubber seals. Just below the bore seal is where the piston wears through the zinc. This is where a rusty/old fluid build-up occurs and your caliper now sticks. To be more specific, there are two major areas that rust and bind. 1. Above the bore seal and just below the dust seal. This is usually caused by a torn or cracked dust seal. 2. Around the bore seal itself. This is from the piston wear and then from sitting and hygroscopic build-up.

This is why your calipers have to be plated once you pop the pistons out. The bores don't need to be, and SHOULDN'T be honed. Honing creates a completely bare steel bore. We all agreed that most brake fluids are hygroscopic... water "will" re-enter you system and it "will" begin to rust. We repaired a customer's 914-6 caliper that he rebuilt himself. He did a decent job. He replated the calipers and they looked good. There was an internal clip issue but, once we got inside, we saw that he had honed the bore. He rebuilt them one month prior. They were already beginning to rust. If you're rebuilding your own calipers, my very best advice is GET THEM REPLATED. The ATE factory used yellow zinc.

Back on topic, once you get that build-up, fluid is just about the only way to get the pistons out. To answer the question of "how water"; we use a hydraulic foot pump that pulls from a reservoir. The fluid is caught in a tray, filtered and goes back into the reservoir. This can pop a piston out in 4-5 seconds. It's not economically sound to build such a contraption for garage use. Just get a grease gun and a couple cans of brake cleaner. IMPORTANT: Make sure you get all of the grease out of the fluid passages. We've had a few cores come in that we didn't know were "greased" and it makes for a non-functional caliper.

Also, if you happened to break a dust boot flange, it probably was going to happen anyway. The cocked piston only "enlightened" you earlier to a problem you may not have known existed until you tromped on your binders.

I keep meaning to do a caliper build thread... oh how I long for those 34 hour days!
Eric Shea - PMB Performance
We Restore Vintage Calipers
Old 07-28-2015, 04:21 PM
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