As I drove back from school an hour ago, I was forced to apply my brakes rather firmly (not slam them, but definitely a firm push). My rear passenger wheel locked up and skidded. The rest of the wheels were nowhere near lockup.

This has been happening for a while, anytime I use the brakes hard. Any ideas? My brakes and rotors were brand new last year, and still work smooth as glass. The calipers had a little bit of rust on the pistons, but were not seized.

My brakes have always been soft, but I assumed it was due to the ABS; until recently, I had never driven another 240 with ABS. But now, I'm guessing something else is wrong. I'm ready to replace my rear calipers if needed, but my father seems to think that the symptoms indicate air or moisture in the fluid instead.

Thoughts?
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'93 244: 'A' cam 4 deg. advanced, 25/22 sways, custom heim endlinks, poly bushings, and a lot more styling customization than I care to recount.