The valve pulses when it purges, so it doesn't allow a solid vacuum.
What I did was disconnect the vacuum line from it, started the car, and rev it up. At certain engine operating conditions it will open and you will feel a machine-gun type pulse vacuum with you finger. Can't remember the conditions so try it cold and warmed up and change the engine speed.
If it doesn't pulse, then I would check the resistence and the wiring.
Here's the link to download the Service CD to find out the particular specs and conditions.
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