Hello.
I'm doing rear rotors and pads on an '89 Iroc with aluminum single piston calipers. The parking brake mechanism is preventing the piston from sliding back far enough to get the room I need for new pads. Is there a kind of star wheel I'm not seeing I have to release? Thanks

 

Sounds like the piston is seized. Real common with aluminum caliper housing across all makes, not just Chevy. Steel piston + aluminum housing + never changed brake fluid = internal RUST.

 

My piston is able to move until it hit that pin coming through the caliber from the parking brake mechanism. I thought that pin had to go back more for space but I managed to wiggle it together.
This is a weird setup with the bunny ears on the pads and the little springs on the piston

 

I was having the same problem when this past weekend when trying to replace my driver side rear brake. I came across this information, I will be trying it this weekend. Let me know if this works for you or not.

the rear calipers have a mechanical type parking brake. the parking brakereplies on a screw-like mechanism and thus you cannot simply compress the piston back into the bore with a c clamp. instead you actually need to screw the piston down. this can be accomplished with either a set of needle nose pliers and some patience. Alternatively a tool exists that accepts a 3/8 drive ratchet and has appropriately spaced prongs that mate with indentations in the piston face. i picked out of said tools up at my local princess auto (unsure what the American equivalent is) for $5. It says that instead I need to use special tool #J-23072 or equivalent.

 

Hello. I got it that day, there's precisely enough room to fit the caliper with the new pads on the new rotor if you line everything up perfect without touching the parking brakes.
It's a perfect fit though it takes some wiggling and getting the angle right.
I compressed the piston fine with a C-clamp, the top plate doesn't spin and it works the threads with the clamp the same way the brake fluid would from the other side.

edit for clarity: I compressed the piston back as far as the pin from the parking brake would let me without messing with it. Then I put in the pads, keeping the caliper mount flush with the ends of the pad and that slipped over the rotor, after a lot of wiggling.
I made this thread when I didn't think that could fit over the rotor without getting more space from that pin to compress the piston farther back,