Many owners with years of service of their Gray, Blalck and Vent valves find them more difficult to operate. Mine got to that point and I learned that the problem with mine and others on the internet is not with the cables, but with the slde valves themselves. I have attached a DIY for the effort that is a simple fix that can be done almost for just your time and likely tools and supplies you already have.
Hope you find the below helpful.
GJ
DIY - Sanitary Drain Valve Cables.docx