What you actually need to use is a 11 mm open-end wrench to remove the air gauge. It must be a very thin wrench otherwise you must take a belt sander or grinder and grind one down thin enough to fit. To let the air out of my streamline I simply use the 11 mm wrench to loosen the air gauge and let the air out of the tank which does no harm. I personally am not fond of the idea of using anything to pry the valve pin open for fear of damaging it. If you look closely you will see that there is a very small air bleeder hole in the neck that the air gauge screws into , so that when you loosen the Gage about a half turn air escapes out of that little tiny bleeder hole . There is a Delrin washer at the bottom of that neck that the air gauge goes into. That's the way I do it with my Streamline.
best regards, Chuck