John Grant's query got me in head-scratching mode. I don't know what method the
Ampico factory used, but I envisioned a constant, stable, metered vacuum source
(turbine type) supplying vacuum to a test jig with a valve attached.

On the "output" end (from the valve port normally leading to a striker pneumatic)
would be a "wind tunnel"; mounted in the far end of which, and sealed around the
edges, being a tiny DC computer fan forced by the flow to operate in reverse:
i.e. as a turbine generator. A digital voltmeter connected to the motor's wires
would monitor the voltage generated, indicating the amount of flow.

I don't know for sure if computer fans would become generators if spun in reverse,
but I do know that some DC motors will. I also don't know how barometric pressure
changes would affect things (I'm not smart enough), but I would be careful to have
one visible, and set all the valves during the same.

Gordon Stelter
Athens, Georgia