I'm replacing the front brake pads on a 2006 Civic. I did the right side, no problem. When I tried to collapse the piston on the left side, it will not budge. Any ideas?

Take the cap off the master cylinder reservoir. Use a disc brake pad spreader to force the piston back into the caliper. Some people use a large C-clamp.

While we find your parts, please enter your ZIP Code at PartsAmerica.com

I was trying with a c-clamp and it would not budge. Hopefully taking the cap off will do the trick. Thanks!

I see from your other post that it looks like you need to replace both front calipers, get rebuilts from your parts store, then yes, you will need to do a complete flush and bleed.