If the Master is firewall mounted, (above the brake cylinders / calipers) then gravity will force the fluid to remain in them (cylinders/calipers)

If the master is under the floor then the fluid has a chance to actually flow back to the master and this will cause a low pedal and that first press on the pedal will be needed to refill the cylinders/calipers.

The residual valves hold the fluid back keeping the fluid where it's needed.