Marauder leakage

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 My Marauder has an air leak. That's nothing new for these rifles but the question is: Where's the leak coming from? I removed the gauge, visually inspected the O-ring ( that small one where the gauge assembly screws into the reservoir) and it looks good. I wrapped tape over the threads, reinstalled and put a bit of pressure in the tube..then left it. The leak slowed but it was still there. Repeated the same procedure and put a thicker layer of tape on the threads...same results. I tried the soapy water route over the filler adapter and the gauge assembly..no bubbles but still leaks air. It's been suggested that it could be the gauge itsself or interior O-rings. I don't feel qualified to take the rifle apart. In fact, it scares me. I've been known to have parts left over when I disassemble something...not a good thing. It looks like I need someone who has experience with these rifles to work it over. Does anyone know someone that works on PCPs? Recommendations are appreciated.
  Thanks..........Larry