When performing front brake pad replacement, some techs remove the caliper then compress the piston with large channel lock pliers or c-clamp.  By using an alternative method, you can compress the piston before you remove the caliper and save a little time on each brake job.   

So how do you do that?

It's easy if the caliper has an opening (many do) like the one in the photo below.  Simply take a small pry bar and insert it in the opening as shown.  Place the tip of the pry bar against the rotor, and the shaft of the pry bar against the caliper, and pry out.  Not only does this compress the piston, it helps you judge how much resistance is present in the piston and caliper slides. 

Prying caliper with small pry bar

I usually open the bleeder screw before I compress the pistons on ABS equipped vehicles (so dirt won't push backwards into the system), and this method really simplifies that task.  You can easily pry the piston in with one hand and operate the bleeder socket with the other.

BTW... This won't work on pistons that need to be rotated while being compressed like those found on some rear wheel disk systems.

 What if a caliper piston won't compress in? 

Typically, if a caliper piston does not compress, you have a stuck caliper piston or a collapsed brake hose.  Note:  If the caliper is on the rear and has an emergency brake cable attached to it, then there is a good chance it is not made to compress in the traditional manner, and doing so, may ruin it.  If you have a caliper with the emergency brake built in, then you will need to rotate the piston while gently pushing in on the piston.  Special rear disc brake service tools are made to simplify this task and are widely available at most auto parts stores.  

Restricted Brake Hose

Of all the "collapsed" brake hoses I've cut open over the years, I have always found the problem to be in an area where a metal support bracket is formed around the hose.  You'll likely notice two things when dealing with a restricted brake hose.  First, when you open the bleeder at the affected caliper, you won't see a steady flow of brake fluid from the bleeder screw when it's loosened or removed.   Second, the caliper won't easily compress unless you open the bleeder.  

Stuck Caliper Piston

If you encounter a stuck caliper piston, it could be caused by a piston that has come out of the piston bore or - is slightly cocked in the bore.  This usually only happens when the pads are wore down so far that the piston is allowed to travel beyond the depth of the piston bore.  Another reason for the piston to become stuck is from rust and debris in the piston bore.  If you have already checked for a restricted brake hose by removing the bleeder screw, and a large c-clamp still barely pushes the piston in, then you are most likely going to need another caliper - or - a rebuild kit, if you're the adventurous type.  Many times if you encounter a stuck piston and it has to be forced in with substantial pressure, that internal piston seal will probably develop a leak and will need to be replaced.