…in the waterline.

Having thought through every possibility without finding an answer, today I called the plumber. The solution was simple, and reminded me that false assumptions lead to false conclusions.

I assumed that the system was pressurized from faucet to foot valve, in which case air would have been impossible. However, because of a check valve, the section from pump to foot valve is not pressurized, and as a result, air was being sipped through a stub that used to be used to prime the pump.

Inspired, I watered the tomatoes (nothing planted from seed seems to make it) and took a nice high pressure airless shower.