1. The OUTPUT of that check valve is the Black connection. Loosen it and start the Black Flush. IF water seeps out the check valve is NOT your problem. It is the tank nozzle is clogged.
2. I have NEVER seen or had a check valve get clogged by debris. Possible but in the hundreds I have fixed never a clog. They either are connected in reverse which means they will not work, or they LEAK due to the seals have failed. 99 times out of a hundred, the Tank nozzle is clogged with debris. The ONLY fix is to pull that nozzle and clean it out or replace if cleaning is not want you want to do.
3. The ONLY way the Tank nozzle can get clogged is because you allow the Black tank to stay above 3/4 full for too long and the dirty water migrates thru those small holes and when you dump, that dirty water dries out and the solids dry out and clog those holes. Doug