Originally Posted by ScOoTeR
did you try taking the cap off the brake reservoir? There's not much room in the braking system for fluid, so maybe you're hydrolocking and that is keeping the piston from moving back in.
I've never worked on a quad- Motor Slut is your bet bet for an answer here.
What he said! Take the cap off and try again. Also make sure you are pushing the piston back in evenly, it will jam if you are only pushing on one edge. If that doesn't work then you must be doing something wrong or the piston is seized.