The venturi in the blast nozzle works on the Bernoulli principle. That means a zone of low pressure is induced in the tube, which should draw "emulsified" sand into the nozzle. The bleed valve is how you control the aeration of the emulsion, and only works because the "mixing chamber" is long and horizontal. If you don't allow enough air in, the emulsion will be too dense to flow. I would start with the valve open and close it slowly while holding the trigger, and stop at the point where it really sings. It should be a set it and forget it type of thing. If you change media, say from silicon carbide to glass beads, you will need to re-calibrate the bleed.
