When replacing the rear brakepads. The piston that compresses on the pads cannot be forced back into the caliper. I have use a heavy duty C-Clamp, a brake compression tool and opened the master cylinder cap and even opened the brake bleeder and the piston remains extended about an inch farther to make room for the new pads.

What would cause this and how do I fix it without going to the dealer?
I tried removing the caliper from the brake lines but the line needs to be disconnected further upstream since the caliper connector side does not allow rotation of the fitting.

 

The rear caliper piston needs to "thread in" as you compress it. Were you able to try that? The C-clamp will work, just tape the end where that little thing just flops around, so it will spin WITH the threaded rod.

 

I use a pair of needle nose pliers. Just stick the ends in the 2 holes on the caliper face and turn it while you push down. Others have used a combo of the needle nose pliers to turn and a C-clamp. It took me a little while to figure this one out myself. I actually had the calipers in a vice at one point...I decided that this was not a very good idea, so I tried something else.

 

Harbor Freight makes a cheap tool to accomplish this as well. You're able to twist and push with it. And it comes with a variety of all the popular piston head "keys". I think it was like $10 or something.

 

HF tool can be had for 20 most of the time.
Well worth the money.

 

I just did my brake pads and used a 9" C-clamp. It took a little adjustment to keep the plate on the piston but once it started it was a piece of cake.
'Thanks to OctaneGuy for his write-up!