Curt, do your input lines (going into the valve) have a key adjustment? There is one for H and one for C.
If so, these may be of help. You have to adjust them in some circumstances and also bleed the lines of air.
After I sat down and thought about it I realized I had drained the lines to work on the shop plumbing. Are the key adjustments on the incoming vacuum valves, back flow preventers, I've gone through them for maintenance. Once the air was purged I was fine. Had a nice developing session.
Everytime I find a film or paper that I like, they discontinue it. - Paul Strand - Aperture monograph on Strand