Have you worked on brakes much before? Sometimes it takes a good amount of force. And you need to be sure to push evenly on the piston so it doesn't bind up (don't just push on one side of it). I use a big channelock pliers with electric tape wrapped thick around the jaws. And I put it so it is even on the face of the piston. You can also get a special tool with two plates that spread apart. Are you working on the front or back? The back are a little different than "normal" in my opinion.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra