if it happens with just the e-brake Its not a return valve problem. The check valves (34-33-1-151-532) are cheap enough so I would replace that just to eliminate the question. I don't see how anything to do with the e-brake can effect the way the motor runs at all. The brake circuit gets vacuum from the engine so I thought that was your problem. If jack is at the end of his rope you should see if he has an extra pump around to install and see if it fixes the problem. If not the problem doesn't sound familiar to me, maybe someone on the board can help you.