If the brake caliper piston will not go back into the piston bore do it need to be replaced.

you need remove 2/3 brake fluid from master cylinder either use large pliers or large Clamp compress piston back into caliper bore be careful dont damage caliper piston seal when compressing piston.

Get a repair manual for your car. Libraries also have an extensive car repair section. Disc brake pads are pretty easy to replace, if you have proper tools (You'll also need a large c-clamp, like 4 or 6 inches to push the piston back into the caliper bore.Use jack stands and pull wheel off. remove caliper(usually two bolts) pull old pads off(may be some retaining clips or wire springs holdin them. Reset piston back in bore, attach new pads, and mount back on caliper bracket. recommended to use some hi-temp brake grease (just a thin film on the caliper retaining bolts, and a dab on caliper sliding surfaces [it moves side to side]).Wont go over the rotor? Your piston is not fully seated. When you've done both sides, before you move the car, be sure to pump the brake pedal a couple of times to push the piston back out, else you will take off and wont have brakes till you pump them.Good luck.

FIRST REMOVE SMALL AMOUNT OF BRAKE FLUID FROM THE BRAKE MASTERCYLINDER. THEN USE A LARGE C - CLAMP AND LEAVE THE OLD BRAKE PAD IN PLACE ON PISTON SIDE THEN COMPRESS THE PISTON IN CALIPER BORE. IF YOU SEE SIGNS OF BRAKE FLUID LEAKING AROUND PISTON SEAL.THE BRAKE CALIPER NEED REPLACING.WHEN YOU FINISH BRAKE JOB NEW BRAKE SHOES IN PLACE MAKE SURE BRAKE MASTERCYLINDER COVER BACK ON MASTERCYLINER THEN PUMP BRAKE PEDAL UNTIL NEW BRAKE PADS SEAT AGAINST BRAKE ROTOR.CHECK BRAKE FLUID LEVEL IN MASTERCYLINDER MAKE SURE ITS NOT LOW.

YOU NEED TO COMPRESS THE CALIPER PISTON USING SPECIAL TOOL 09580 - 3400 OR YOU CAN USE A LARGE C CLAMP AND THE OLD BRAKE SHOE NEXT TO CALIPER PISTON.BE SURE TO SIPHON OUT 1/3 BRAKE FLUID OUT OF THE MASTER CYLINDER BEFORE COMPRESSING CALIPER PISTON BACK IN THE CALIPER BORE.WHEN YOU FINISH REPLACING REPLACING REAR BRAKE SHOES.MAKE SURE BRAKE MASTERCYLINDER RESERVOIR COVER BACK ON RESERVOIR BACK IN PLACE.THEN PUMP BRAKE PEDAL TO SEAT BRAKE SHOES AGAINST ROTORS AND UNTIL YOU GET FIRM SOLID BRAKES.THEN CHECK BRAKE FLUID LEVEL.DONT DRIVE CAR IF BRAKE PEDAL FEEL SOFT NOT FIRM.CHECK FOR LEAKING BRAKE HOSE OR LEAKING CALIPER PISTON SEAL.

to replace rear pads on a focus,you need to remove the caliper assembly remove both pads you will notice that the caliper piston has a grove, that is where a special tool locates because the caliper has the hand brake incorporated in it and has a thread in the back of the piston you need the tool to put pressure and turn the piston to move the piston down into the bore of the caliper to allow for the extra thickness of the new pads then , make sure that the piston grove is in the right position because the pad locates in that grove so fit pads back into caliper make sure your slides in the caliper are moving freely so you dont get any binding of the pads refit caliper do the same on the other side but the only thing different is that the piston turns opposite ,you will see if you are turning the wrong way the piston will try to turn out of the bore and you need it to turn in .BE VERY CAREFUL when the job is complete and you are ready to test drive the vehicle you need to pump the pedal until you have a full and solid bake pedal, because you need to push out the caliper pistons before you have a solid pedal and then you will need to top up your brake fluid at the master cylinder and your ready to go regards agwbk

YOU NEED LARGE C - CLAMP TO COMPRESS PISTON IN CALIPER BORE.REMOVE SMALL AMOUNT BRAKE FLUID FROM BRAKE MASTER CYLINDER RESERVOIR FIRST BEFORE YOU COMPRESS PISTON. AFTER REMOVING A LITTLE BRAKE FLUID.AND COMPRESSING CALIPER PISTON IN BORE.PLACE MASTER CYLINDER RESERVOIR COVER BACK ON.WHEN FINISHING REPLACING BRAKES.PUMP BRAKE PEDAL UNTIL BRAKES FEEL SOLID AND FIRM CHECK BRAKE FLUID LEVEL.DONT DRIVE VECHICLE IF BRAKES DONT FEEL FIRM OR SOLID.

• To install: Front

1. Compress the piston of the disc brake caliper.
   
2. Install the brake pads and caliper assembly.
   
3. Lubricate the guide pin. Tighten the guide pin to 16-23 ft. lbs. (22-31 Nm).
   

REAR :

1. Pull out the pad springs and then remove the pads and shims.
   

To install:

1. Clean the piston end of the caliper body and the area around the pin holes. Be careful not to get oil on the rotor.
   
2. Carefully turn the piston clockwise back into the caliper body (this can be done with a pair of needle nose pliers). Take care not to damage the piston boot.
   
3. Coat the pad contact area on the mounting support with a silicone based grease.
   

A special tool is available to rotate the caliper piston

WARNING

Do not press the piston into the bore as performed on the front disc brakes. Due to the parking brake mechanism, the caliper piston must be turned into the bore.

WARNING

Do not press the piston into the bore as performed on the front disc brakes. Due to the parking brake mechanism, the caliper piston must be turned into the bore.

The problem lays with the Caliper itself and/or the brake hose connected to the Caliper.
However if you replaced the pads, did you also replace the Rotors or have them Turned? The old pads wear the rotor. New pads on old rotors that have not been replaced or turned may end with rubbing or stuck brakes.
A simple way to test whether it's one and/or the other:
1. Remove the Caliper from the rotor, remove the pads. Keep for now the caliper attached to the brake hose.
2. Very slowly push on the brake, exposing more of the piston out of the bore. Not all the way. Usually until the rubber dust seal/boot is fully extended.
3. Check the seal/boot for cracks and tears, and if clean or not. Bad seals may prevent the piston from re-seating.
4. Using a c-clamp and pushing straight in: Try repushing the Caliper Piston back into the Caliper Bore (the cup back into the hole). It should go back in realitively easy.
5. If it doesn't go back in easy: Again slowly pump the brake and re-push the pistons back out to full extended seal/boot (but not the piston out of the bore).
6. Detached the brake hose from the caliper.
7. Again using a c-clamp and pushing straight in: Try again to repush the caliper piston back into the bore without the hose attached. If it goes back-in relatively easy - the caliper is okay...it is the brake hose.
8. If the caliper piston does not go back in easily - Replace the caliper.
9. When Installing the new (reman) caliper, remember to bleed the brakes.
TRY EITHER OR #10 OR #11 BELOW:
After the new Caliper is reattached to hose and has been bled:
10. Again push on the brake petal to fully extend the caliper piston fully (rubber seal/boot fully extended) Again do not push the piston out of the bore! Try pushing the piston back into the bore. If it does not re-seat relatively easy: Replace the brake hose.
11. Another method: After replacing the new caliper back on the rotor: Assumng the entire front end (2WD front wheel drive) or entire vehicle (2WD rear wheel drive) or (4WD all the time) is jacked up off the ground
a. Put the lug nuts back on the rotor.
b. Have helper Start the vehicle and place in Drive. Don't step on gas!
c. Have then let off the brake and then engage the brake.
d. When they let off the brake watch to see if the Rotor is turning or not, if rubbing or not. Or if still sticking.
e. With a new caliper, turned or new rotors, and still a problem? It is the brake hose!
12. Replace the brake hose and try again.

Another method but more expensive:
OR Replace the calipers, brake hoses; bleed and test!

If this helped or not; or if you need additional help or have addtional questions let me know on fixya.com!

nice car i have one myself but the back brakes are a little bit tricky because you can not just push the brake calipers piston back into the bore. but here is how you can do it. you need a large plieres and you have to put the head of the pliers into the calipers piston then spread apart the handels so that the head of the pliers opens up against the inside walls of the piston. then all you have to do is Turn the piston back into its bore. See the rear caliper piston on the mustangs are just like a bolt. they are threaded into the bore and when your pads were they screw them selves out so when you redo your brakes you have to screw them back in just like a bolt. I hope this helps you out on this and if you have any more questions about this procedure just let me know and i would be more than happy to assist you in this.
please rate me if you would
thanks
Aaron

while the seals in the calipers are replaceable, its probably easier just to get a new/re manufactured caliper. if for any reason the seals don't work perfectly you could lose control of the vehicle when attempting to stop. with that said i'll try to explain the best i can how to replace the seals in the calipers for that vehicle.

1. jack up the front of the vehicle, place on jackstands and remove the front wheels, make sure the parking brake is on
2. unbolt the caliper and unbolt the brake line from the caliper and take caliper to a clean bench
3. remove the dust boot retaining ring on the caliper that holds the boot to the caliper itself
4. using a blowgun with a rubber tip apply low pressure compressed air where the brake line was bolted to the caliper to push the piston out of the bore (be carefull and ready because the piston may shoot out with some force)
5. when the piston comes out you will need to replace the dust boot and the seal so remove the boot and seal carefully so you don't scratch the piston ( the seal is actually in the caliper piston bore hole)and throw out the old seal and boot.
6. inspect all the surfaces and make sure there are no imperfections at all on the piston, the piston bore, or the caliper, if the surfaces are marred in anyway, replace the caliper, if they are good, using brake cleen and compressed air clean all parts then dry them
7. get the new seal and lubricate with DOT 3 brake fluid
8. install new seal into the caliper bore
9. apply a thin coat of DOT 3 brake fluid to the piston itself
10. install bottom half the the caliper piston into the caliper bore.
11. install new dust boot seal onto the piston
12. compress the piston to the bottom of the caliper bore
13. fully seat the dust boot seal into the caliper and install retaining ring
14. reinstall on vehicle (make sure to use a NEW copper washer on the brake line to caliper fitting) and bleed the braking system while checking for leaks.

repeat process for the other side.

if the caliper leaks at all from the piston after seal replacement, replace the caliper.

make sure you properly bleed the brakes before attempting to drive your car.

*these direction were extracted from personal experience and manufacturer information*

any other questions just ask!