Probably don't need rebuilding but do need cleaning.
Buy some spray brake cleaner, remove caliper, pump the brakes to push the pistons out a little and then clean around each piston with the brake cleaner spray and an old toothbrush. Push pistons all the way back in and re-assemble. Don't forget to pump the brakes up before going for a ride.
If the pistons are really hard to push back in (ie you can't do it with your hands) then they need a rebuild.
Better yet, do the conversion to 4 piston calipers and say goodbye to poor brakes.