1994 Plymouth Duster BRAKES RELEAS VERY SLOWLY / SEEM TO DR

Brakes problem
1994 Plymouth Duster 6 cyl Front Wheel Drive Automatic

My son has a 1994 Plymouth duster / sundance. When you apply the brake it seems like the back brakes come on first, then the front. Once you stop, when you take your foot off the brake pedal, the car won't move. Then slowly the brakes will release. I bleed the brakes front and rear and there is no air in the lines at all. Could this be a porportion valve problem or perhaps a power booster problem? I'm not sure and unfotunately I don't have a repair manual yet. Please help! Thanks. JC