On S-drills/valve drills breathing down a reg will not let water in through the second stage unless you press/bump the purge button after removing it from your mouth. It may get bumped/pressed while it's off, but that's not part of the drill
Prior to going to the reg again, the gas (should) be on so hitting the purge is a non issue at that point (and is preferred anyway as it is the method of checking the reg is on prior to going to it).
If ya muck up the drill, well then it's all bets off
As noted, emptying the hose not only ensures you've properly located the reg leak (It will stop) and also will allow for (a chance at) re-seating a first stage o-ring. Of course this may allow some water in through there.
In reality if we had a first stage o-ring leak pre-dive (especially yucky saltwater) we would ascend and reset there so as to be as kind to the gear as possible. We've done it here in fresh water while underwater however.
regards
EDIT: Ah I see handsome Jeff beat me to part of it.
Prior to going to the reg again, the gas (should) be on so hitting the purge is a non issue at that point (and is preferred anyway as it is the method of checking the reg is on prior to going to it).
If ya muck up the drill, well then it's all bets off
As noted, emptying the hose not only ensures you've properly located the reg leak (It will stop) and also will allow for (a chance at) re-seating a first stage o-ring. Of course this may allow some water in through there.
In reality if we had a first stage o-ring leak pre-dive (especially yucky saltwater) we would ascend and reset there so as to be as kind to the gear as possible. We've done it here in fresh water while underwater however.
regards
EDIT: Ah I see handsome Jeff beat me to part of it.