[chuckster57]

If the gray and black both dump out one drain and both valves were open with the cap on, then yes eventually the tanks will equalize levels.

Are you figuring the black isn't draining because the panel says 1/3 or 2/3 full? If so, chances are there is toilet paper draped across the sensors. Hook up the black rinser. Shut the valve and stand at the toilet with the toilet flush held open. Watch for water level and have someone watch the panel. When it reads full, if you don't see water in the pipe at the toilet then the issue is with the sensors.

Continue to watch for water at the toilet, and then open the valve when you see water backing up to the toilet. If that doesn't clear the lights then you may have to repeat. Lots of people say the sensors are a joke and don't work anyway. I have never had sensors that don't work properly, just have to keep the tanks clean.