I am not sure of the specifics of overhauling that hand pump.
when you lift the cab, you send more fluid to the base end of the cylinder per inch of travel than is returned from the rod end because of the amount of fluid the rod displaces.
the hand pump being sealed should go into a vacuum and draw fluid from the AOP as soon as you send out more fluid than is being returned. It get fluid from the AOP via that 1PSI check-valve I circled in the above drawing. If the hand pump case, the filler plug or even that control valve body was not able to pull a vacuum, or that 1psi check is clogged, the hand pump might have trouble making up for the difference in cylinder flow to-from when lifting the cab…
it is way easier to suck in air than draw fluid, if it is drawing in air, that air displaces fluid and keeps the hand pump reservoir from refilling and it will send fluid to the AOP with each cab lift/lower cycle…