Break Pads on 2000 DTS

I've got a 2000 DTS and the rear break pads on the passenger side have worn. We have not checked the others, but we cannot get the new brake pads on. The old ones come off and go back on just fine, but the break piston will not go all the way back into the callpier. And therefore we cannot get the new pads on.

Any ideas or special tricks that must be done in order to get this to work properly. Thanks.

Not sure here, but the piston must rotate as it is pushed in ,special tool used for this normally, should also open the bleeder put a suitable hose over it fill a small jar with brake fuild and submerge the hose in the fluid as you retract the piston, if you are unfamiliar with all this get it done by a pro.