When I checked mine, I went "air over hydraulic." I made a setup similar to Bill/Red58's description. I used a bicycle inner tube filled with water connected to one of the sides of the valve. Then I used a bicycle pump to apply pressure. I could regulate the pressure pretty easily this way.
Mine leaked, so I bought a really crummy reproduction (do a search for "TxLaTx crabbing about heater control valves" and it'll probably light up) and re-jiggered it to work. I kept the old one and will play around with changing out the NAPA BK660-1000 grommet that Red58 mentioned later this year.
While I'm on the soapbox, here's another thing I came across: I put my old valve in some 5% vinegar and that baby came out looking VERY good! It dissolved all of that gunk that had been accumulating for 50 years. Just don't leave it stewing for too long or it could end up damaging the metal joints. After pickling it and giving it a good rinse, I lubed the mechanism with moly grease and it seems to work very well. Prior to the dip, it would barely work even after applying a lot of force to it.
Bill
