A friend who just arrived at a dive venue has water intruding into his housing, an older Subal D7ii model.  He thinks the leak is around one of the optical fiber ports, which does have an o-ring.  The housing repressurizes in 10-15 minutes, so whatever it is is slow.  He's done the usual o-ring service and is pretty obsessive about it.  I suggested a dye test with food coloring, which I don't think could hurt.  Any insights or suggestions?  Thanks!

32 minutes ago, RickMo said:

  I suggested a dye test with food coloring

 

Yes just a bit and fill up well the housing with white absorbent paper. You should find exactly the point or, at least, the area.

Yep, as Davide says. Fill the housing with soft paper, loo roll works well, and then leave it submerged for, say, an hour. Then check for leaks. The paper will be wet close to a leak. If still dry, submerge again for a longer period. 

Thanks, Gents.  I've passed it along and will report what I learn.

Rounding back on this—he identified a leaky ring at the vacuum pump, which only needed cleaning.  He’s a little bummed—he thought he had a golden ticket to mirrorless.  Thanks again for the memorable advice.