Yesterday I rebuilt my 1.5 inch brass gate valve. I knew the air was getting in the valve stem (sprayed water on it and the bubbles would go away.) Tip learned here!
So anyways it was a little issue that was bugging me. I took of the handle, unscrewed the packing nut, and the bonnet and removed the stem. Cleaned the stem with a wire brush, then applied liberal amounts of silicone plumbers grease to the stem and the threads that the gate screws into, packing gland and all the threads.
Put it back together, tightened things down and no more air bubbles in my skimmer basket. It took about 20 minutes max. Hardest part was getting the handle off and getting tools together to do it.
Valve looks like this: http://www.nibco.com/assets/T113IR.pdf
Should be good for 10 more years!