There's probably a "crud ring" on the piston that needs cleaned-off. Take the pads back out of the caliper, GENTLY give the lever just the tiniest of squeezes to push the piston partially back out of the bore (a small piece of wood might help you not to "overshoot" it), then use Brake Parts Cleaner and an old toothbrush or brass-bristle brush to thoroughly clean the piston(s) of any spooge. Crack the bleeder-screw, then push the piston back-into the bore.
Carefully inspect any pins for corrosion/straightness when you reattach your new pads.
Bleed the caliper with fresh DOT3 or DOT4 (I like the DOT4 even in DOT3 applications - it's compatible, but NOT the other-way 'round!), and you're done.
It goes without-saying, you should protect your eyes during any/all operations where some chemical or debris might be flung-into them. That Shyte Hurts.