You cant, it needs a 'C' clamp.
if it's like older jetta's or most imports you need a special tool{or a channel lock pliers} to turn the piston back in
The calipers should turn back in with a big scredriver or such! you have to rotate the piston to retract them rotae the pistons clockwise with a large pair of pliers
Have noticed that Ford still uses the screw style pistons on the rear calipers. To retract the piston use either a caliper/piston tool or a set of needle nose pliers. Turn the piston clockwise to get it back in but take note of the position of the grooves as they normally need to align so that a imaginary straight line through the grooves points to the center of the caliper.
the rear pistons have to be screwed in to get them back into the caliper. this is because the emergency brake is built into the caliper you can rent a caliper return tool or be careful with waterpump/channel lock pliers and turn the piston, it will start to retract into the caliper when you are done make sure the grooves in the piston are in the correct position 12 and 6 o clock as you will notice the is a pin on the inner brake pad that must seat into the piston. once job is done pull the e brake up and down 10 or more times this will return the pads back to adjustment
You need to turn (screw, rotate) the piston clockwise in the caliper a large number of times to make it fully retract. You can insert some kind of tool into the indentations in the face of the piston, or grip the very top edge of the piston with pliers in order to turn it. It has to be fully retracted to allow enough space for "new" pads to be installed.
Shade tree mechanics use a C-clamp and a flat steel bar to compress the piston. I have also used a large adjustable Channel-Lok-type pliers to compress.
there are two ways caliper pistons are made, one you can just use a c-clamp to push the piston back into the caliper; the other piston type must be screwed into the caliper with a special tool (inexpensive and readily available at your local auto parts store), but i must warn you, this "screw type" is very annoying to deal with, sometimes you have to start screwing it in with a pair of channel lock pliers b/c the tool wont fit in the caliper until the piston is most of the way back into the caliper, but be careful not to scar the outside of the piston and not to break or tear the seal
---- You have to turn it (caliper piston) as you are depressing the piston back into the caliper. You can't just "jam" it in. I used a caliper tool (for depressing the caliper) and turned the piston with large jaw pliers. This gives both inward pressure and the turning motion. I turned it clockwise and that seemed to work. There is a special tool you can buy to turn the piston as you push it back into the body of the caliper but if you work carefully and you are VERY aware of the rubber boot (don't rip it) then pliers are ok (don't tell the Ford dealer). Cover the pliers with something so you don't scratch/ruin the piston. The front brakes are just a standard push - don't need to turn them. Why do the back brakes work this way? Has to do with the parking brake. If you look you can see that it is attached to the caliper on the rear.
Same as the front except don't try to compress the caliper. remove the cap that is in back and loosen with an Allen wrenchAfter you've installed the pads and put the caliper back on, retighten that screw in the back of the caliper with the Allen wrench until it is tight again, then back it off a quarter turn. This is to readjust the brakes. If you don;t do this the brakes will be really mushy.AddendumThere is no Allen key or bolt to undo (unlike a Saab 9000). Instead, when the pads have been removed, you need to turn the caliper piston to retract it.Use a pair of pliers direct on the piston and turn clockwise to retract. This works!
on a mazda3 rear caliper there are two dimples in the piston, you can use needle nose pliers to turn the piston while you push on it, or you can rent a caliper compression tool from a local auto parts store, if there is a harbor freight in your area you can buy the tool for around $20. using a c clamp will damage the caliper, it has to have pressure and rotation to be compressed
Correct tool to use Brake Caliper Piston Rewind Tool A lot of pistons easily go back in using a lever or pair of large oil filter pliers just take care not to damage piston seals and not to damage the caliper surface.
Remove wheel; remember to block up front wheels as you need to take hand brake off. Remove wheel, undo 2 x 19mm bolts from rear of calipers and remove caliper off the disc, remove old pads, compress piston. The rear brakes have a slotted piston that needs to be turned clockwise to retract nd push piston rearward back into caliper. There is a special tool available at local autoparts store or you can try needle nose pliers. Replace pads and put caliper back on disc asnd bolt up.