Try either a big C-clamp or a big pair of channel locks, and squeeze the caliper piston back in. To get the other brake pad out use either of the tool above, and squeeze on the lip of the bake pad and the caliper. If you can't do it with a c-clamp or a pair of channel locks then your caliper is probably seized. I have had some luck getting them to brake loose and work fine for a couple more years, but if they are seized it's probably time for new ones, either now or down the road if you can brake them loose.

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