You may just need to adjust the stop on the steam valve; you need to take the top off of the machine to expose it, then loosen a hex-head bolt, tighten the valve then re-tighten the stopper. This may be preventing the valve from turning far enough to close. You may also have some calcium build-up on the face of the valve piston which is preventing it from sealing properly; a descaling (with citric acid) should help.