The collection chamber should not fill with water. I'm not familiar with the particular model, but your description sounds like a fairly typical venturi. If the air inlet is closed, you get no air and therefore no foam and you're just pushing water through a tube. Try opening up the air inlet until you have a head of bubbles below the collection chamber. The bubbles pick up the goop from the water column and kind of build up into more of a foam until they reach the collection chamber where it will collapse into a nasty brown stinky liquid. Depending on the setup, this might just collect there or it might drain via a tube to a separate jug. The more air you let in, the less water. The trick is getting it adjusted to produce the best foam, which takes some playing over time. You generally don;t see foam right away. Also, with my venturi inlet, I get a scale build up over time that I have to soak clean in vinegar to keep air coming in.
It is a violation of federal law to use this product in a manner inconsistent with its labelling.