As others have said, you may well be able to loosen the lock nut and screw in the tee handle a couple of turns to solve your problem. My trailer provides that adjustment, I'm nt sure if they all do.
Another possible issue might be the operating rod arrangement.
Again, I don't know if yours is set up like mine, but the operating rod going to the black tank valve has a ninety degree bend in it. Because of this, when you push the tee handle in, you are not getting a direct push on the valve operating rod and things don't work as smoothly as they might.
If the valve on mine mine is at all baulky and doesn't want to close well, I reach in behind, grab the portion of the operating rod after the ninety degree bend and push directly there as well as on the tee handle - that gets it the valve closed nice and securely.
(PS - not sure I would be trying hydrochloric acid! But if you do, and if you want to dilute it, add acid to water, not water to acid when diluting. If you do it incorrectly the acid can spit up in your face! Safety glasses recommended in any case!)
Brian & Connie Mitchell
2005 Classic 30'
Hensley Arrow / Centramatics
2008 GMC Sierra SLT 2500HD,4x4,Crew Cab, Diesel, Leer cap.