open atmosphere fluid tanks?

[TheBennyB]

So I'm a definite newb to hydra. Been wondering about this and researching for a bit. How come the Eagle Mach dozer and 870 have an open air and still get pressure? Both have essentially "over spills" off the fluid tanks stock that would allow air to both enter/exit. Is fluid level essential in keeping balance?
For me not. Just cruise till I hear pump motor differnence (higher rpm.) Man, sorry for the back to basics question, but I don't get it, far as I'm concerned it's voodoo like electricity or folding a fitted sheet.