If the washer goes back exactly as it was before, and the nut is tightened very hard, it will not leak. But, it likely will otherwise. So a new washer is always a good idea. I use the Milwaukee regulator assembly too, and it works adequately well for me, but I wouldn't recommend it to anyone not willing to fix the various leaks, replace the solenoid, possibly not use the bubble counter due to leaks, etc. Tom is correct - spend a bit more and you can get a lot better quality.