I know this is an old thread but I am just repairing an S400 with a gauge O ring leak and encountered the same problem. The reason the gauge doesn't register is the bottom O ring on the securing bolt is blocking the threads and preventing air reaching the gauge drilling. It's caused by too small an O ring or tightening it down far too hard. It is a very poor setup and the weakest part of the gun. It really needs a small Doughty washer as standard. I still had to buy a new guage anyway as it was