[Lil Giants]

Originally Posted by frizzen

If you don't run it vented at atmospheric pressure, you'd have to keep a big enough head pressure above ambient, with enough volume, on the reservor to keep the pump from pulling a vaccum on the reservor and making the pump cavatate when fluid is drawn out and sent to the cylinders.
When fluid is out in cylinders that volume isn't available in the reservor, pump, or return lines.
Even on a vented reservor, it's still sitting at 14.7 psi, plus weight of fluid column above pump pickup, for head pressure to supply the pump.

(My experience is mostly 3000psi aircraft systems charged with 1000psi head pressure, not construction models.)

Great explanation Frizzen!