How to free a stuck brake piston?

July 9, 2008 8:28 PM

Car-repair-filter: Any mechanics got a trick for freeing a stuck brake piston?

So I decide to replace my brake pads this afternoon. I do 3 sets and everything's fine. When I get to the last wheel though, the pads are worn way down, and one of the two pistons on the caliper is funny looking. (I couldn't tell you how it was funny, it just looked odd, for some reason I can't put my finger on.)

Anyway I pull the old pads, grease the shims, slap the new ones in there, and get to the part where you have to push the pistons back into the caliper. The first one went in fine, but the funny-looking one only went in about halfway, and wouldn't go any further.

I tried using a C-clamp and some cardboard to push it in, and opening up the bleeder valve, but the thing still wouldn't budge. So, it's stuck, somehow, probably cocked sideways in its hole. I put the old pads back on (cause the new ones wouldn't fit in the half-open caliper), bled the lines and it drives fine.

I understand that I'm probably looking at doing a caliper rebuild (blahhh), but I'm wondering if there are any trick-up-the-sleeve kind of things that might allow me to free the piston without taking the damned thing apart. If it matters, this is a 1997 Toyota Supra, ABS, two-piston front discs. Thanks!

posted by sergeant sandwich to travel & transportation (8 comments total)

The only thing I have used is a little compressed air to force the piston out. If your going to do that, your doing a rebuild (seal kit is under $20). Sounds like the seal is probably gone and there is rust or other contamination stopping the piston from going in straight.

my 2 cents.

CDM
posted by Country Dick Montana at 8:38 PM on July 9, 2008

Compressed air works -- but you need to wear eye protection. Every so often something goes wrong and the piston flies out like a bullet.
posted by Forktine at 9:20 PM on July 9, 2008

As long as you're sure the "funny-looking" piston isn't connected to the emergency brake you can do this:

1. Take the caliper off and clamp the piston that's free with the C-clamp. If you had to disconnect the brake hose, re-connect it.
2. Put a wood block in the way of the "funny-looking" piston or leave one of the brake pads in there.
3. Step on the brake. The only piston that will move is the funny-looking one.

You should make sure that you have enough brake fluid in the reservoir before you try this, but the hydraulic pressure from the brake system will shift anything that isn't welded together.
posted by jet_silver at 9:54 PM on July 9, 2008 [2 favorites has favorites]

I will second CDM's point on a seal kit. You need to unstick the piston but then the job is half finished. Then you need to address the reason why it got stuck in the first place, which is usually rust/corrosion/contamination from a bad seal.

Bleed the brakes when you're done and don't forget to pump the brakes before you try to move the car. That can be an easy step to forget when you finally triumph over a stuck piston.
posted by KevCed at 12:15 AM on July 10, 2008

If it's a rear brake caliper, and includes the parking brake, you need to screw in clockwise while pressing in. Of course, you should have had to do that with the other rear brake caliper as well. Step 7 on this page shows what I'm talking about, though I always just use a c-clamp and channel locks instead of the special tool.
posted by Dorri732 at 3:30 AM on July 10, 2008

You didn't ask, but if you're rebuilding 1, you should certainly rebuild 2, and probably rebuild 4, as you'll be doing some bleeding anyway. I always used brake cleaner and a toothbrush to get the walls nice and shiny.

PS your car is awesome and I am jealous.
posted by Kwantsar at 9:19 AM on July 10, 2008

okay, i'll try jet_silver's thing tomorrow.. i'm assuming the block of wood is to keep the piston from coming all the way out? if it doesn't work, i'll do the rebuild this weekend and put up with my landlady's nasty looks.

it's the front driver's side wheel, so no, not the e-brake, and it's definitely not one of the kind you have to turn to make it go in either.

thanks kwantsar, i'll try to remember that next time it's raining and the sport top is leaking.
posted by sergeant sandwich at 3:25 PM on July 10, 2008

Sometimes there's a little air in with the fluid, especially when you're in the middle of brake work. That makes the braking system partially pneumatic, and the compressed air stores energy you don't want to recover in a sudden burst. The wood block keeps the recently-freed piston from shooting off into the weeds. You can finesse this a bit if you know how far the piston will come out without leaking, just select the thickness of the wood block.
posted by jet_silver at 10:06 PM on July 10, 2008