The knob is just a valve, to control air flow through the jets. It's commonly referred to as an air flow control valve, or afc. There should be no water in the plumbing of this valve while the jets are running, although once the jets are off, the afc lines will fill with water.
There's a
venturi at each jet fitting, creating a vacuum. The water rushing over the venturi creates a vacuum in the air line. These air lines run to the afc valve (your knob), which when open allows air to be sucked in.
When these lines are purged with water, it's not because the afc has failed. It's becasuse the venturi isn't working properly, and instead of a vacuum in the air line, there's water pressure. This normally happens when one of the jet's, or some portion of one or more of the jet's is loose. You don't need to drain the tub to tighten or replace these parts of the jets.
If you can post a pic, maybe someone will be able to point you in the right direction as to what you'd be tightening or replacing.
Hopefully helpful,
~Swine