95 chevy brake booster problem??

I am working on a '95 Chevy w/ a 305, 1/2 ton 2WD. Booster went out recently, replaced it, prob. went away. After a week truck came back with a different problem, operator says when he comes to a stop sign (sometimes) the pedal will not come back up and the brakes will not release. Give the trk some gas and it will barely move, let go and it will come to a stop by itself just like if you were braking. The prob. is intermittent. I have not been able to "catch" it in the act. Until recently that is, by the time I get to where the trk is, the pedal has released. I have noticed after a week in the shop, that the pedal will release when you kill the motor and then press on the brake pedal again. Could the new brake booster be the culprit? Any help would greatly be appreciated, thnx. L8RS