I use one of the old pads placed against the piston and a large c-clamp to push the piston back into the caliper's bore, it's very easy to do.. No need for anything else. Just make sure you have the cap of the master cylinder removed when you push the piston back in and drain pan or a piece of cardboard underneath to catch the fluid. I have an 87 and believe it is the same as your 91. It is very easy.
I hope you are also replacing the hardware too. The wedges shouldn't be reused.
It would also be beneficial to rough up the rotors with some emory cloth if you are reusing them before installing the pads.