.... when there is a leak (loss of quantity indicated) then , there is no chance for pressure !!!
Depends on the system, where the system is damaged and how much is lost. From memory, the 767 has standpipes in the reservoirs which save fluid for certain critical user systems.
and even when the pumps are off, when there is a leak, it leaks everywhere inside the aircraft....
Everywhere inside the aircraft?
...and at minus something for hours hydraulics can freeze or damage other systems when the fluid is not contained in a tank/pipes
Freeze point of Skydrol <-62.2C. Freeze point of Jet A fuel -40C.
Do you have any examples of these other systems?