One company did offer a plastic tool that was suppose to help. Basically had the shape of the air tube radius, with a slight raised “button” in the middle to fill the gauge port space.Never tried one….got use to poking the o-ring “belly” back in place with a shaped wooden dowel (actually was a chop-stick cut down shorter for a fatter end, end slightly concave). Silicon grease...go slow...poke down more than you push.If you want to get a little ambitious, it’s the sharp edge of the hole….if it had just a little radius on the inside edge, would be less of a problem.Sounds like I do a lot of this…but a lot of Crosman PCP’s use the same type of gauge block (not the same part...but the same design). Disco itself hasn’t needed all that many rebuilds, but the more PCP’s you own that use the same type of gauge block system, the more practice you get at o-ring poking.Likely part of the reason I start leak testing with a bare sealed tube is so I can leak test again after it’s “fixed”, Finding a new created leak only after full reassembly really tends to tick me off.
Being “Captain Obvious” for a second...just to be sure…..looking at the Crosman parts diagram….the 18F o-ring that’s hidden inside the valve (sealing the two halves of the valve body). Can get over looked as you can’t see it. IF that one leaks, it’s behind the tube sealing o-rings (18D) and would leak out the back end of the rifle (and the front screw hole of the trigger housing).