Pontiac Repair: rear brakes, chevy lumina, pontiac grand prix

Question
Can you please tell me what size hex wrench I use on the top bolt on the rear brake caliber. And is there something else
I can use to take off the back brakes pads.  My problem is that I cannot get the bolt out of the top of the rear brake caliber and the part that the auto parts store is selling me is not working.  What do you recommed.  I have changed these brakes b4 and never had a problem like this.

Thank you
Michael Wright

1996 Pontiac Grand Prix SE.  

Answer
The bolt has an 18mm hex head that holds on your calipers Michael. There are a bunch of things that could be giving you grief with the service of these brakes. First and formost that particular vehicle, dubbed the GM "W" body along with Chevy Lumina and Buick Regal to name a couple more, the rear calipers went through about 6 different designs due to several flaws and have an extemely high failure rate so they may need replacement anyway. Even if they are ok, they need to have the piston rotated, not just squeezed, back into the bore with a special tool. The other big failure point, which may be your other problem , is the bolt/slider rusting and freezing. Try penetrating oil on it. Hope this helps and good luck. :)