Purging the Oil lines is one issue, Sticking Hydraulic Tappets/Cam Followers/Lifters is another.
Once you purge the oil lines and the engine starts and displays oil pressure for more than 5 seconds, the Oil Line Purge is complete and was successful.
Oil pressure = Oil hoses purged of Air.
Hydraulic Tappets are essentially a slightly leaky oil pump with no outlet.
On the downstroke of the Cam Lobe, the spring in the Follower causes it to expand and take up any slack in the valve train.
As it expands it draws in oil filling it up.
On the upstroke of the Cam lobe, the pressure caused by the valve springs causes the follower to leak a tiny amount of oil past its piston.
The piston is a very close, but not perfectly leakproof fit in the follower.
This leakage allows the follower to continuously adjust its fit within the valve train as the temperature of the engine changes.
The free play on a closed valve will typically be well less than 0.001. Way smaller than you could achieve with manually adjusted lifters.
When the engine stops, the lifters that happen to stop with their associated valve in the OPEN Position will eventually leak all the way down.
On the next start, these lifters will quickly refill themselves and return to normal within a few seconds.
Any air that may have been ingested by a lifter will quickly be purged from the lifter by the leak down process during each valve opening.
A brand new operational lifter with no oil in it whatsoever might take a minute or two to fill but not much longer.
If you have a clicking lifter it is most likely because of a malfunction of the fill valve in the lifter causing the lifter to lose its oil as fast as it takes it in.
It is not an Air problem that has suddenly occurred. Air will come and go. A clicking lifter will need hands-on maintenance.
