Solved: How to find vacuum leak in water supply line?

Hi everyone,

so we are back home and testing.

Should there be any type of seal or gasket between the strainer and pump?

Strainer looks like this, screws directly onto the pump inlet as one unit: SHURflow RV MARINE Water Pump Inline Stainer Filter 255-213 / 255-313

There is no seal there, just hard plastic flanges. Is that correct, or is there something missing? Reassembly with vaseline briefly fixed the symptoms, which may mean I am looking in the right places...