Black Tank Dump Valve

When I dump a full black tank, the valve opens very easy, no problem. I flush the tank for 5 to 10 minutes, then I close the valve for 5 minutes to let it partially fill. When I then pull the handle to open the valve again, it is so difficult that I almost cannot open it. Been this way for a couple of years, and now I thought I might do something about it.

I put tank valve lubricant in the tank periodically, but that doesn't seem to make any difference. Any ideas why full it is a piece of cake to open, and after flushing and partially filling, you almost need a gorilla to open it.