I don't think you can get a realistic measure of valve seat leakage by
applying pressure from the top.  A valve will seat better with vacuum
from the bottom.  Pressure from the top will tend to unseat the valve
whereas suction will help it to seat.

Under-suction is the normal operating situation; so this is how leakage
should be measured.  I measure flow through an open valve with pressure
from the top.  But I measure leakage with vacuum from the bottom.  To
measure a single valve's leakage, you have to isolate it, which can be
difficult.  You may have to make a suitable fixture to apply suction to
the bottom of the seat.

Spencer Chase