Had a similar problem off and on for years and would either take the valves apart or replace them and they'd work for awhile and then get real stiff and hard to operate,not seal properly and eventually fail. I'd been using either Vasoline, or plumber's grease on the seals as recommended by the rep at Thetford. Found out both of these products are petroleum based and will swell rubber seals. I then realized that I was having similar problems with faucet seals and other similar rubberized things I was applying these lubricants to. I was told by a plumbing supply store to use silicone grease instead because it's a non petroleum product and will not affect rubberized seals. I thought back about my college chemistry classes where we were told that " like will dissolve like". For example, water based paint needs water for cleanup and not paint thinner. Maybe not a good example but I think you get the idea.
So, I recently had to replace both dump valve seals again and this time used silicone grease as a lubricant instead. This stuff is kind of hard to find and a little pricey but you don't need to use much. Haven't had a chance to take the rv out yet so I can't report the results. Also, I really never had any problems with the cables hanging up or anything
