Hi CD Eldorado,
My black tank valve is at the end of a long 7' cable too, and it really helps to lubricate both ends of the cable with
Tri-flow. Spray it on the closed black-tank valve, and when it is open, spray the now-exposed components of that valve again. Also, while it is open, spray the exposed cable end at the pull-handle. Be careful to not breath the fumes.
I believe the valve itself is just fine, but if the cable hangs-up, it can keep the slide-valve from fully closing.
Why is all of this reminding me of that scene in RV?
Thanks, Eagle5