Zippsy:
Just curious - I've never heard of annual service for BC's. What do they do? My first BC lasted around 13 years and never had anything done to it other than a good washing after each dive. My wife's lasted longer without any servicing either.
Depends on the BC and the manufacturers reccomendations...
The spring that holds the air dump valve closed could get rusted or weaken for some reason so it may need replacement once every several years is one example I can think of.
With a well maintained BC, this "shouldn't" be an issue, but then again...
If the spring IS shot, it could ,in theory, fail mid-dive and then your BC would not hold air.
OR it could simply not hold the valve closed as tightly. Then your BC would work just fine as long as you didn't put too much air in it... EG: the bladder is rated to 35 Lbs of lift but since it won't hold as much pressure, it may only lift 25-30 Lbs before beginning to vent air.
The inflator hose and valves and the bladder itself are also gone over (for wear and tear etc and any lifeforms inside, etc...) as well as the outer shell for holes, tears, etc...
Like I said, alot depends on the type and make of BC.
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And now, just because I can, I'll type it one more time: "etc..."
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Having said all that, I've never had anyone other than me tear apart my BCs and clean/reasemble them yet and they seem to be in perfect working order...
The only thing I'd be curious to find out is what the 'correct' pressure before venting on each one is and how close to that pressure my BCs really hold nowadays.