[busterbrown]

I'm pretty sure its a straight drop like on my Bullet 308BHS. In fact, as Chuckster stated, you can peak down with a flash light and see the bottom of the tank when it's empty. My blank tank has a plastic ring (probably scrap during assembly) in it that circulates around when the fluid levels are high enough.

As a way to provide a visual to the black tank rinsing process, I will disconnect the city water connection as my blank tank is flushing via a different water hose. I keep the black tank dump valve closed and will hold the foot pedal down on the toilet. With a flash light, you can see inside the tank as it fills with fresh water. When the level is just about at the top of the tank, I open the gate valve and let gravity do it's thing. Make sure your sewer connection is seated properly as the blank tank "dumps" much quicker than than the gray tank.