"You've got an air leak at
"You've got an air leak at the strainer, and the culprit is that whimpy rubber gasket.
Here's what I did and it works like a charm: Make an O-ring by getting several feet of 1/8 vacuum tubing from NAPA, along with some rubber cement. Next, cut a length of tubing that fits tightly inside the strainer. Select some electrical wire that you can slide inside the tubing--a snug fit is what you're looking for. Coat the wire with the rubber cement and stick the wire inside the tubing. (The wire length needs to be about 75 % of the tubing length, with the middle of the wire at the joint.) Be sure the ends of the tubing are cut nice and square, and that the cement bonds them tightly together.
Finally, since the "O-ring" is thicker, you need to shim the screen lower. Unscrew the nut at the bottom and put a thick flat washer on the shaft (above the screen).
Good luck!
Jeff"