The 22 has a foot pump--and just a plain tube for the water to come out over the sink--no valves there. The valves in the Whale foot pump are fairly robust, but it is possible that is an issue. There is also no strainer, at least in any of the 22's I have owned. It is possible that some previous owner put a wire mesh filter somewhere in the line, and I would look for that.
I would also follow the lines to be sure there is no link or something pressing on the lines. I also would shock the tank with chlorox bleach. (about 1 cup for the 20 gallon tank full--or 1/2 cup for 10 gallons. Let this sit for a few hours, and then pump it out--rinse several times to be sure you have all of the bleach out.
For the sight tube that many of us have: a .22 cal gun patch soaked in Chlorox , on a cleaning rod makes a great way to clean out the sight tube,