On some rigs, it's really tough to replace one of the two valves due to very limited space. This can cause you to not get one or more of the rubber boots sealed properly.
I ended up just removing the entire "manifold" which contained the "Tee" where both black and gray valves are mounted. I rebuilt the manifold, replacing both valves... lubed them up really well... and then was able to reinstall the manifold/valve assembly as a completed unit.
Whatever the cause... we just have to get those black water leaks fixed.
Best of luck.