If it's the rear brakes you're talking about, do not try to compress
the piston with channel locks or a C clamp. You will need the special tool that enables you to screw the piston back into the
caliper. The tool is available from places such as Sears, Snap On, Matco, Mac, Napa, and other parts suppliers where tools are also sold.
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Re: Rear Disk Brakes
You press it in using a variety of tools: a special tool is actually made for this, but: I use a large pair of channel-lock pliers, but I place some plywood against the pistons first so not to scratch. This is actually a good time to "exercise" the pistons back and forth nealy full stroke because it helps to prevent corrosion inside the cylinder walls. Watch the brake fluid reservoir since pressing may overflow it! Rate me solved...thanks!
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Remove the caliper mounting bolts and remove the caliper. Replace brake pads as necessary. Special tool is required to retract caliper piston to fit over new pads. (see picture below) Retract piston in caliper using special tool to rotate clockwise and make sure the notch in the piston is aligned properly so the tab on the inner pad will fit in properly. Reinstall caliper assembly and caliper mounting bolts.
YOU NEED A PISTON COMPRESS TOOL TO PRESS PISTON BACK IN THE BORE WITH THE BLEEDER SCREW OPEN ON THE CALIPER*.CLOSE BLEEDER SCREW JUST BEFORE THE CALIPER PISTON IS PUSHED FULLY INTO CALIPER.THIS WILL ALLOW ROOM FOR NEW BRAKE PADS AND CALIPER FIT OVER THE BRAKE ROTOR.WHEN FINISH.PUTTING BRAKE PADS ON YOU HAVE TO BLEED BRAKE SYSTEM.DONT DRIVE VECHICLE IF YOU CANT GET A GOOD FIRM BRAKES.
If you have disk brakes, and you cannot fit the new brake pads in the callipers? Loosen the brake fluid reservoir cap to allow pressure to escape. Use a large C clamp connecting it to the front and back of the piston. Turn the screw on the clamp to compress the brake piston then release the clamp. The piston will remain compressed allowing you the fit the housing over the new brake pads.
The rear disk brake piston does not compress back into the caliper, it has notces on the caliper piston that a tool fits into and the caliper piston is actually turned or screwed back into the caliper, and a notch in the piston must also be aligned to fit the the small protrusion from the backing plate of the inner brake pad.
The brake tool can be obtained from most auto part stores, and it is usually square with protrusions sticking out from the sides in different patterns to fit various caliper pistons, and it usually attaches to a 3/8" drive extension and ratchet.
Do not compress this type of caliper piston because the caliper can damaged.
Normally rear disk brake pistons have notches on the top of the piston that a tool does fit into (usually a small square shaped tool with protrusions that fit into the notches on the top of the caliper pistons and this tool usually fits onto a 3/8 drive extension and ratchet), and the piston is then rotated down into the caliper, it actually screws into the caliper. You do not want to compress this type of piston or you will damage the caliper and piston.
Use a "C" clamp. Position on the piston and when you tighten it up it will compress the piston in the caliper. Remember to loosen the cap to your master cylinder, as you will be forcing brake fluid back into it. When you remove the "C" clamp the caliper will stay open long enough to align on the rotor.
Most auto parts stores have a little cube that fits on a 3/8 ratchet wrench. use this cube to screw in your caliper. it has various nibs on it to fit a number of brake calipers. costs about 10 bucks. screw it in clockwise until its almost flush to the caliper housing...if you screw it in too far...the boot will pop over the piston and you will have a bigger mess. sometimes a screwdriver can be used to turn it but i prefer not to bleed. lastly make sure the grooves on the brake caliper piston is horizontal as to allow the caliper to slip over any "Nibs" on the new pads that are for alignment purposes.
On the rear calipers you need to screw the piston back into place. There is a tool nicknamed a rubik's cube. That will plug into the piston and you hook up a 3/8 inch ratchet and screw it back in while pushing inward. http://www.summitracing.com/parts/WMR-W80621