Rock Creek mentioned drilling a hole in the bottom of the elbow to let it drain so it won't freeze.
I would doubt 6' would start a siphon with some air in the system but would be optimistic about 15'.
lassig,
To get the rest of the air out of the intake pipe you either have to:
put a check valve in it
put a valve you can close in it
vacuum out the air, vacuum pump or the vacuum of a gas motor
suck the water into the pipe with a water pump with the suction hooked to the vent
wait for a flood
If you use a threaded coupling you will get very wet getting it unscrewed and it will be pretty tight with the pressure on it. Should work several times, last couple threads might get damaged with repeat usage.
