Instructables

Step 8: Compress piston

Since your new pads are obviously much thicker than your worn-out pads, there's not enough space to put your new pads in place.

So, you need to compress the piston back into the caliper.

There are many ways to do this; probably the safest is to use a C-clamp around the piston and the back of the caliper.

I couldn't do this, so I used a GENTLE prying technique between the caliper and the piston.

Be careful not to scratch, mar, or bend your brake disc. And be careful to exert pressure *straight* on the piston, as much as possible.
 
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crazytech246 years ago
On a vehicle with ABS you never want to simply push the piston back into the bore. This will cause dirty fluid to enter the ABS solenoids and cause damage. Some ABS systems are completely integral and can be VERY expensive to replace. When pushing the pistons back into the bore you must open the bleeder and SLOWLY apply pressure to the piston. If you dont have a Vacuum?Injection Pump, its best to have someone there helping. Make sure you do it slowly and DO NOT let air into the system. Once this is done you can top off the system.
jrj906207 years ago
I've read that it's better to open the bleeder to let out excess fluid to prevent dirty brake fluid from going back into the system when you push the piston in.
jkohns8 years ago
Note that on the rear disc brakes of some newer vehicles a combination of TURNING and PRESSURE is needed to compress the piston. This generally requires a special tool that can sometimes be found at online auctions or auto parts stores, but is not cheap. Also, brake fluid can eat your paint job, and should never be allowed to simply 'spill out'. A $2 turkey baster will take enough off the top to prevent spilling.
0.775volts8 years ago
I don't actually siphon fluid out of the resivoir, I just take the cap off, that way it'll just spill out, and you don't have to worry about removing too much fluid (though I've never had it spill, one cylinder at a time isn't very much fluid.) also, when bottoming your piston, i use an old pad across the piston, and put the c-clamp on that, it helps to distribute the pressure across the surface of the piston, and generally keeps things more in check. I highly recommend you get a pair of jack stands, they're cheap, and the only true safe way to work under a car (unless you have a lift handy). your car's emergency jack was designed for just that : an emergency. don't do anything more than change a tire with it. you'll notice the author has a heavy-duty floor jack, which has a bigger footprint and is much more stable.
jswilson648 years ago
Before compressing the piston, siphon a bit of brake fluid out of the master cylinder reservoir to make room for the fluid you're pushing back up the line. If you have to top the reservoir off later, use new brake fluid.