Our Valterra valves are about 22 years old and still function . My black tank valve does leak a small amount , so I installed a third and final valve .
Anyway , when the valves start feeling 'stiff' , I spray lube and probably more than enough . Some times it doesn't last a long time and others it seems to be fine a lot longer .
Back flushing seems to help alot .
With the third valve in place and open , I first dump the black . Then close the third and open the grey and let it back flow into the black until something close to equal . Then pull the third for a final drain .