The air passage and paint path never cross within the brush, so a blockage in either would remain isolated. Air can only be introduced to the paint path, and as a result back into the cup, at the front of the brush. It is usually the base of the nozzle where air finds it's way in. So, it is usually either the nozzle, or nozzle cap. The nozzle must protrude from the nozzle, as least a small amount. If not, air will find it's way in to the nozzle - rather than over it. There must be a gap between the nozzle cap opening, and the nozzle. It is not a large gap, in fact the smaller the better - but, it has to be there. It should also appear to be an even gap all the way around. No gap, and instead of the air rushing out the front, it will be forced into the base of the nozzle, as it has to go someplace. A crack in the wall of the nozzle tip will cause an uneven distribution of pressure around the nozzle, and force air back into the paint path/cup. On a rare occasion, a dirty nozzle will also cause an uneven distribution of paint exiting the nozzle, causing high pressure air to be drawn back into the nozzle - bubbles in the cup, many people will swear they resolved it by cleaning the nozzle, but the actual occurrence of this situation is uncommon. It is usually one of the other areas that were resolved in getting to the nozzle to clean it
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Teflon tape is not a good choice for trying to seal any of these areas. On a brush that may have wider tolerances in spec, teflon tape may be thick enough to throw things out of wack. Use bee's wax, or chapstick for the easiest and most efficient seal.