I also have a 2000 maxima and I just put new rotors and pads on it. I found a really helpful tool at Checkers for around $9.00. The tool has the tip made to fit in the grooves on the caliper piston. The tool fits on a regular ratchet and makes adjusting the piston a breeze compared to using the needle nose pliers I used before I got the tool. The adjustment tool is also designed for use on other vehicles as well, so it's well worth buying. Hope that helps.
I have a 2000 Maxima SE and encountered the same problem. I could not get the rear caliper piston to retract. I tried draining fluid, removing cap from master cylinder, no parking brake, etc. I have not tried the solution suggested, but I will tomorrow morning. The solution/suggestion was found on Nissan Forums Bulletin Board at http://www.nissanforums.com/archive/index.php/t-90044.html To make a long story short, this is a quote from that page:
"You need to turn them back in. Get a needle nose pliers put int the groves and turn them back. I least that is what I have to do on my 99 Max"
Like I said, I haven't tried it yet, but will give it a try. Hope this solves our problems!
UPDATE: This is taken from the Nissan Maxima Service Manual:
6. When installing new pads, push piston into cylinder body by gently turning piston clockwise, as shown. Carefully monitor brake fluid level because brake fluid will return to reservoir when pushing back piston.
7. Adjust the piston to the right angle as shown in the figure.
In other words, adjust the piston until the concave portions of the face of the piston are at the 12, 3, 6 and 9:00 positions when looking at the caliper with the mounting bolts parallel to or level with the ground. The convex, raised portion of the face of the piston should be in an X position. I hope that makes sense. Now I know I'll definitely work on this in the morning. Good luck.