black water clean out connection leaks

The ones the manufacturer installs are junk. You need to replace it with a better one. I think it has to be installed at the high point where the current one is installed. Not sure it will work properly if installed outside. Although it kind of works like a check valve, it is not a check valve. If you Google vacuum breaker you can find a lot of cut-away views.

I'm not sure why they don't just use a regular in-line check valve but I bet someone will tell us shortly.