VP shunt valves prior to implantation are tested with a manometer setup as follows:

What you see here is a three way stopcock, manometer (vertical plastic tube marked in increments of centimeters), an adaptor for the outflow nozzle of the 3 way stopcock, and the shunt valve. The switch on the three way stopcock is turned so that the shunt valve is closed. A syringe filled with saline is then attached to the inflow used to fill the manometer. The switch of the three way stopcock is then turned so that the valve is closed.

At this point the column of saline in the manometer will drain out of the distal end of the valve.