you need a purpuse made tool to wind the pistons back in and can be awkward especially if you've never done them before but if your carefull they can be wound in with the spanner used to tighten the disc on an angle grinder.however if theve gone past their stops there is an o ring which seals the unit and you have to make sure its returned correctly as your brakes will leak and they will probably require bleeding
paul
August 2008