how do you collapse the caliper's on a 2008 grand prix

The back yard mechanic way is to pull them off and use a "C" clamp to depress them. Or use a screwdriver and pry them into the caliper.

If trying to press the pistons back by hand. Sometimes they go back in easier by removing the brake fluid resivoir lid.

For the rear brakes you will need a caliper compressor, which you can borrow from Advance Auto. The rear calipers have to twist to retract because of the parking brake mechanism. You can't simply push them by hand or with a c clamp.

Use a big ol' C-clamp. Put the screw end of the clamp right into the piston. Just watch for brake fluid to come out of the master cylinder reservoir. That's normal, but can make a mess and the fluid will take the paint off anything it splashes on.

Are you guys sure about all this C clamp business? Don't all modern GM calipers need to be turned and "threaded" back in like the ones on my '06 Cobalt? If that's the type they are just trying to push them back in won't work. Of course I could be wrong. I'm sure a quick trip to You Tube would provide the answer though. If they need to be turned back in there is a nifty tool you can buy for that purpose from the auto parts store. If I recall correctly though I think I just improvised with a good pair of needle nosed vice grips.

NEVER try to use a C-Clamp to compress the piston on a Grand Prix REAR Caliper.. I have a 2005 GP GT ABS, and ONLY the FRONT Brake caliper piston CAN be compressed with a C- CLAMP.. To compress the REAR Caliper piston, you have to use a tool called"Four Wheel Disc Brake Piston Tool", which rotates the brake pistons back into the caliper cylinder. Can get one from Harbor Freight for $3.97. Use with a 3/8" ratchet with an extension..