Great, but when the fluid cools down again it will then create a slight vacuum - drawing in outside air with its humidity. What then?

 

That is the nature of the system. I believe but am not sure that the vent hole in the cap is slightly offset from the hole in the bladder/gasket so there is no direct route to the reservoir fluid. The absorbtion of any moisture and contaminants through the air above the fluid over a period of time is one of the reasons Polaris recommends changing the fluid at the intervals scheduled in your manual.

 

Yep, same old crap. The Polaris front master cylinders are notoriously known as junk. There should have been a recall on every unit sold since they started production. Same goes for the rear brake cylinder. I've owned 3 different Polaris's (2 were new) and every one had POS rear brakes that would leak down and need to be bleed over and over, not to mention the rear brakes alone will not stop any atv they have ever made.