My backup reg did purge, as it still had hose pressure - should I have purged longer to avoid this problem?
Purge should return sufficient information if you'll be able to breath from this reg when you need it - you can feel if the pressure is going down or 2nd stage is supplied with continuous flow.
In an actual OOA situation, does this still stand, or is it only a drill procedure while you're learning the skill?
No, in OOG you go for a backup without any purging. If you notice that you left post is closed at that moment (e.g. during V-drill when your left post is closed), you simply open the valve. If this does not solve the problem then you probably forgot that you islolated and your left tank is empty
but you still have your wing inflator as a 3rd reg.
The drills/simulations I did included me feeling/looking at both sides & looking at my SPG after first isolating (ie before closing posts or switching regs), and my instructor then signalling either SPG pressure drop (indicating left post leak) or gauge steady (indicating right post leak)
Realisticly, any leak that can cause visible pressure drop on SPG will be detected before with hearing bubble stream. It's faster and more productive to concentrate on audial detection then to go for SPG.
The GUE link posted differs, and said to isolate only after the reg switch - I was taught to isolate first as part of the diagnostic procedure (see above). Assuming you prefer the GUE method, can you explain why?
I can understand that it's safer that way for a drill, but does it teach you the diagnostic skill you will require? Or perhaps that is something that is taught later with GUE in a more advanced drill?
First of all, don't mix V-drill with failures resolving because those are two different things. Drill does not teach you diagnostic skill. Drill is a drill: reaching valves, being confident with valves turning direction, being aware of valves status, being stable in water column, being aware of the environment, performing drill in safe manner, etc.
Diagnostic and resolving is taught at Tech-1 level and above. Just a few thoughts about it although this subject has been addresed several times. We don't go blindly to any post because then it's not diagnostic - it's pure guessing. The goal is to find proper solution as fast as possible. The fact is that highest chance for failure is at right post - if we feel bubles at right side we go for right reg first (switch, team attention); if bubbles stop, that's it; if bubbles continue we isolate. Isolating first can save few seconds in case of burst disk problem or one side manifold problem but will give no information in diagnostic process.
it's a double fault:
