There is a plug on the back of the caliper that you have to take off and back the screw out inside it. Otherwise the piston will NOT compress.
NO...sorry, this information confuses everyone searching later on...
the REGULAR proteges share this design...there is a set screw in the back of the caliper that is used to retract the piston for a pad change...the MSP rear calipers DO NOT have this...
As mentioned...you just need a brake tool that is a little metal disc that turns the piston, and most parts stores carry these for less than $5...I bought one at Napa years ago, and is a has a 3/8" drive in the center...if looking to buy one, you need one that is not much bigger than a quarter, which is usually the smallest they carry (they get MUCH bigger, and a big one will not work)...i've gotten away with just using some small snub pliers and pushing while turning, but its definitely not the most comfortable way to do it...the right tool has it retracted in seconds...
also, not sure what company originally developed this...but its very common on single piston slide caliper designs...and is FAR superior in reliability, simplicity, and sealing than the set screw setup in the back...
lastly, also as mentioned...make sure the cut outs in the caliper piston are straight up and down...the pads have little notches on them, without lining those up the piston won't sit properly on the inner pad...it will only take about a turn and a half to fully retract it, after that it will just spin freely and not retract any more...its really not that big of a deal...