Compress rear brake caliper on 1996 nissan maxima

Study this to understand disk system
Go to pep boys or wal-mart and get a can of 'PB Blaster' - accept NO substitute. While you're there get a c-clamp who's mouth about as wide as the caliper.
Remove caliper from bracket and remove pads. If it's stuck to th disk you may have to force/pry it. ABSOLUTE last resort spray w/PB.
Do not remove the brake line!!!
With caliper away from disk remove pads and spray area where piston(s) enter caliper w/PB. Wait about 5min. Put brake pad over piston(s) and c-clamp on pad and back of caliper. Slowly tighten clamp all the way until pad is flush with caliper. 'wala' should be free!
An alternative is to remove caliber and have someone pump the brake, using hydraulic pressure but I do not recommend this: A - can easily pop piston out making a BIG project B - although 'free' the piston/seals are not lubricated
Now the real problem is why is the piston frozen? Lack of use you should be OK. But if brake fluid is not clear you need to change the brake fluid - not for the weak of heart!!!!
If you have to do this I strongly recommend buying caliper rebuilt kits (about $5 ea) and may even consider rebuilding the master cylinder while you're at it. It's pretty easy once you get everything apart.
After all brakes are the most important part of your car!!

Compressing Rear CalipersIf you open the "bleeder valve" you'll need to bleed the air from your break system. As your breaks wear down, the level in yor break resivor will drop. what works for me is the following: Remove the cap on your resivor and place some shop raggs around the base. With a large "C" clamp push the piston back/into the break caliper (slowly) when the piston is all the in your resivor level will come up. When you complete both sides, you resivor should be near the corect level. Remember that when you put it all back together, your break pedal will go to the floor. Do this a few of times and your break pedal will tighen up. Now you should check the level in the resivor.

Hope this helps and be safe.....