I have stock calipers that I fixed up and cleaned out years ago, and they still work better than a new quad. The trick is to make sure the pins that the pads ride on are spotless, I hit mine with a wire wheel to knock the rust and dirt off. Also, if you pop the pistons out of the calipers and take a small pick and pop the o-rings out that the pistons seal against, you can get in there with a pick and clean any gunk out that's building up behind the o-rings. If those o-rings have gunk behind them, it's going to make them squeeze the piston tighter, and the pistons won't want to retract and your brakes will always want to drag.
Almost forgot to mention, I ordered up some Russell Speed Bleeders years ago to help bleed the brakes. They were a little expensive, but it was super easy to bleed the brakes.
