halocline
Contributor
I have tried that test but hooked up to my tanks with the air off. Seems to form a vacuum is why I was thinking it was the exhaust.
If you have done this, and the regs truly hold vacuum, like you cannot pull any air through them at all while they are attached to a tank but not pressurized, then there should be no way for water to leak in. Are you sure you are not getting any air at all when you try to inhale on them during this test? Just for grins, try it with your mask on; maybe the mask is somehow making your upper lip not seal against the mouthpiece. It's a long shot, but it only takes a few seconds to try it.
Other than that, the only thing that is different in the water is the exhaust valve opening to let out your exhaled air. When that happens, virtually all 2nd stages let a small amount of water in, but since the exhaust valve is typically at the lowest part of the case in normal dive position, it usually just leaks back out on the next exhalation. If you turn upside down, i.e. facing the surface, then the mouthpiece flange is the lowest part of the reg and any excess water in the 2nd stage will drain into your mouth. This is a design issue. You can replace the exhaust valve (diaphragm) and carefully clean the case where it seals; that might help some. It's not uncommon for a little sand or dried salt to disturb the seal there.
But, I will say that I have never found any 2nd stage of any design to be 100% dry in all positions, and I have never found any 2nd stage that really holds an excellent vacuum under hard inhalation pressure to breathe noticeably wet.
If it's the sort of thing that is very noticeable and seems to be independent of position, and your 2nd stage is doing a good job of holding vacuum on your test with the tank turned off, my best guess would be a mouthpiece issue, meaning your mouth is not sealing around it consistently.