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

Check pump strainer....might not be tight or might be cracked
Also check pump casing for any signs of cracks

Sucking in air can only happen on tank side of pump and same symptoms using winterizing kit then YES issue is between that section and pump itself

With pump OFF

See if you can 'fill' suction side of line/strainer/pump using winterizing kit and then place TP around fittings/clamps/connections/strainer---pump casing and look for wet spot (might have to loosen strainer a bit so water can run in/fill up line)