Cleaning the back of a brake piston

The problem:

You're going to replace the brake pads, but when take the old ones out you notice that the pistons look like crap. They're coated in grey crud. They look like this:

original - 88.5 kB

original - 72 kB
In the first picture you can see that one of the pistons has been pushed out a bit, and the piston is nice and shiny in the bit that was behind the dust seal. In the second picture they've both been pushed out a bit.

The problem is, the new brake pads are thicker than the old ones, so the pistons have to be pushed back into the caliper. This is bad because you don't want all that crud scraping up the seals or even worse, finding its way into the brake system. You've got to clean the pistons off.

First you want to push the pistons out enough so you can get at all the crud. I take a wrench or screwdriver or piece of wood and put it in the place that the brake pads were, so you can pump out the pistons with the brake lever without having to worry about one of them popping out and getting brake fluid everywhere. Don't worry, they won't come out evenly. When they're both showing some shiny piston, clean the crud off with a brash brush. Some people use a toothbrush and brake cleaner or Simple Green® or the like, but I like the stiffer brissles of a brash brush. It probably doesn't matter.

So, now you've cleaned the bit you can see, but there's the other 7/8 you can't get the brush to that still needs cleaned!

You want to rotate the pistons, here's a trick:

Pump the pistons out far enough so you can grab onto them with chanel-locks and twist them around so you can get to the other side.


I use a couple pennies, they're hard enough to be useful and not allow the tool to push through, but softer than the piston so they won't damage it. Be careful so nothing touches the pistons but the pennies. If the wrench slips and you hit the piston, that could be really bad. You can also use a couple layers of old soda/beer can if you like. I get warm fuzzies by using something thick, like pennies, and I usually have a couple in my pocket. I don't drink beer out of cans except Guinness, and I don't drink soda, so those options are less useful to me.

If you don't protect the pistons, the tool will damage them, the dents/scrapes will destroy your seals, and you'll end up with a brake fluid leak which is a hassle to fix and quite dangerous if fluid gets on the pads or rotors.

Don't screw around here.

Here are some pics of what I mean about the pennies.

original - 101 kB

original - 70 kB

original - 63 kB
Make sure all the crud has been cleaned off, and there's no rust, dents, grooves, etc. If there are, the party line is that they should be replaced, but lots of people have good luck filing and sanding pistons with a fine file and sandpaper. I'll leave that part up as u an exercize for the reader.

Once you've cleaned the pistons and they're nice and shiny, push the pistons back into the caliper. Put a wrench in the pad place and grab onto it with your fingers, then push on the caliper with your thumbs. The pistons should slide into the caliper. If your old pads were really worn you may get some brake fluid coming out of the resevoir. Yuk.

Oh, the missing ceramic plug on the face of the piston fell off, and I didn't put it back on for the pictures. They can be held on with a tiny dab of RTV silicone. Trying to put it back together loose is really hard, so don't bother unless you can't get any RTV.