71-72 Front Brake Calipers...Sticky Pistons?

[chewed 71'er]

Has anyone ever experienced stuck or sticking pistons in their brake calipers? From the amount of rust I cleaned out of the bleeder valves, I think the calipers in my 71 are original. In everyday driving, it stops okay. But stopping in a hurry is a problem, as truck wants to continue rolling, and brake pedal seems solid (but not bottomed out). Sometimes rear brakes lock up stopping hard, doesn't pull left or right. Have plenty of lining on front pads and rear shoes. Recently replaced master cylinder and proportioning valve (for leaks), throughly bled all 4 wheels. Power booster is working properly. I guess there's the possibility of worn rotors?