Strange brake problem

Long time lurker here with the first post. I've learned a lot about the wife's '96 Geo Metro hatchback with the 1.3l, manual shift and AC from y'all. Yesterday after a fairly long, in town, trip (bout 50 miles) a strange thing happened to me. The brakes, which had been workin just fine, suddenly became hard to push and had reduced stopping power. As I went on, the first press of the brakes was fine but if I let up and pushed them again "same thing" hard pedal.

The front disc pads have bout half their life left, brake fluid is clear and full, caliper pistons checked ok (not frozen), brake booster vacuum lines all in pretty good shape..... what else should I check?

Sounds like your brake booster is shot. Check the vacuum line going into the brake booster and make sure it's not leaking.