How do I check the black drain valve?

[JimboFunFinder]

Even though all three valves are fully closed while towing, there is always some residue from the black tank when I remove the cap. I am guessing the black drain valve is not leak tight. Is there any way to inspect the valve? How difficult is repair/replacement? Any suggestions?
Thanks!

 

[Ironbutt]

JimboFunFinder
I had the same problem in my 2015 210UDS. It turned out to be a shaving of plastic, most likely from the original installation, that was stuck in the valve gate. After emptying the tanks I looked into the opened valves (with a bucket underneath to catch drips) with a flashlight and removed the piece of plastic with a long handled needle nose pliers. No more problem. I think when they drill the tank to install the plumbing they just let the debris fall where it may.

 

[JimboFunFinder]

Thanks! That makes sense! After our next camping trip in a couple of weeks I will clean and flush the tank so I can take a good look when I get home.