Interesting. I really don't see why light pressure (less than 5psi) on the air above the fluid would push muck into the m/c or hurt the seals. The seals aren't that puny. I can easily understand it if pressurised fluid from a can was used as you said used to be done because the injected fluid could stir things up in an already dirty reservoir, but that's not what we do.

Re the moisture in the air, if filters on the compressor are good enough to eliminate moisture from the air when using the volume of air that gets used when spray painting, then any water in the tiny amount of air that is used to lightly pressurise the m/c must surely be infinitesimal. I don't think your comment holds.

I've had great success gravity bleeding 70s Corvettes which also have fixed 4pot calipers (front and rear) but never got much volume through on a Volvo, even on one last year where absolutely every line, including the hards, were brand new. New m/c and rebuilt calipers too. Don't know why.
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JohnH, Sydney, Australia