I have recently purchased a 50 y.o. home, split-level, 2-1/2 bath. The toilet in the full bath upstairs "locks" when filling. It may fill 3-4 times or only a few seconds when a "hammer" knock is heard in the line and the water shuts off completely. I disconnect the flexible hose at the valve at the floor, bleed tiny amount of water from the valve, reconnect line and water flows again for as little as a few seconds to > 1/2 hour.
I have inquired at several hardware/plumbing outlets and even consulted a retired plumber with no answer.
All other faucets/toilets/outlets seem to be fine at this time - it's only this one toilet in question.
I'll certainly appreciate any help I can get!

I'm going to put up two ballcocks. Which one is yours? Regards, Tom

There is no such thing as an air lock in a water supply line.

However, you do have something that is blocking the line. It could be any thing. It could be something like a small pebble but is most likely a small amount of solder that entered the line during assembly or when repairs were done.
When soldering a joint, solder can run inside the pipe if excess solder is used. This excess solder can form a bead and run down to a cold section of pipe or it can fall and splatter into a thin flat disk.
Water movement then carries solder to the next available valve. It can take years for the solder to break loose and move.

Remove the valve under the toilet, turn the water on and flush the pipe briefly. Inspect and reinstall the valve. Since the solder may be lodged in the valve itself, I would just replace the valve.

Tom - I have the system on the left, without the ball.

Shut the water off at the house shut, follow Harolds advice and flush out the supply. If, after inspecting the angle and finding it clear reinstall and test. Good luck, Tom