Sounds like you found your problem.
However, just in case, I've also found another thing that causes air leaks.

This has happend to me and took a while for me to figure it out.

Not sure what kind of valve's they are (I think Jandy), but the valves that allow me to control my skimmers and main drain have been the culprit in the past. These valves are the type you have to loosen a little before you can open/close them. Well sometimes, if they ae too loose they allow air into the system. Funny thing is they do NOT leak. So when my filter looses suction after being off for a while and won't prime on it's own, that is usally the first thing I check, and 9 times out of 10 it is exactly that. The other 1 time was that the O-ring on my pump was worn.

I'm thinking this should be a "sticky" as one of the first things to check when a filter won't prime. What do you think?