Air lock on cold feed from tank in attic

[Johnyc86]

Hi experts
Fast one for ye.
Air lock on cold feed from tank in attic.
Got rid of it by cracking open nut on gatd valve.if the airlock was on the elbow just after tank connection how could you get rid of it buy opening nit instead of going into attic and cracking open elbow.
All compression fittings..

 

[Chuck]

Hi experts
Fast one for ye.
Air lock on cold feed from tank in attic.
Got rid of it by cracking open nut on gatd valve.if the airlock was on the elbow just after tank connection how could you get rid of it buy opening nit instead of going into attic and cracking open elbow.
All compression fittings..

Is there an 'inerted-U' or a horizontal run in the path of the pipe? If so, the run needs modifying to eliminate it. There needs to be an upward gradient towards the tank to avoid airlocks.

Post some photos if the cause is not obvious.