Usually part of main (hydraulic)resevoir separate from the rest of the fluid by a stack pipe. This allows the fluid to be used by all services until it gets down to the stack pipe and then only essential services ( like landing gear) can use the fluid remaining.
Called Bootstrap as in "pulling ones self up by ones bootstraps" ie last bit of effort.
As a matter of interest most big pistons had the same arrangement in the oil tanks to reserve the last of the oil to feather the prop. <img src="wink.gif" border="0"> <img src="wink.gif" border="0">