I have a Vetus strainer, and if the O ring seal is just 1/12th of a turn not done up, it does not seal and air gets in the system, which means my engine does not pump water.

I have put grease on each joint in the system to ensure and air tight seal, from cock to pipe, pipe to strainer, and from strainer to pump. Also the O Ring to the filter lid.
It probably wont show a leak as such, as air enters the system.

No problems now.