Easy way to find out. Block the port to your vacuum booster. If you get the same result, you can eliminate that as the culprit. But it seems that you would barely stay running with a leaking vacum booster. Best way to be sure, start the car, run it for a few minutes, then shut it off. if theres a vacuum reserve, thats not the issue. your next stage is to top off the fluids, raise your idle speed slightly, then try and work through the gears. Correct me if im wrong, but if the torque convertor was locked, it the motor wouldnt turn in park