Those heater valves are just not my favorite thing at all. The cables don't actually open the valve, there is a diaphragm that actually opens the valve.
You can clean them, there are kits to rebuild them, but if the diaphragm is no good then there is not much choice except to find a NOS valve. The people who rebuild them won't do anything with the copper diaphragm, last I checked. I took mine apart and thought, well, maybe if I was good at tig welding or braising very thin copper I could fix it, but I didn't know what or how to fill it. Seattle Packard Co. sold me a new one.
More about these valves here, how to test the diaphragm, etc.
Rebuilding a RANCO heater control valveThis Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=250160