91 T-Bird, rf brake lock/sieze up

I have a 91 thunder bird sport, 3.8l, i have already replaced the rf caliper, rf brake hose, and had a power flush/bleed done on it, but the d**n thing still locks up on me. any idea's? according to ford, my car DOES have ABS, but i have never noticed it working, and have locked up on a couple of occations.....if anyone can help, please...do, i'm about to take the car out back and put it out of its missory here soon lol.

Reguads
Tony