I was thinking the same thing, but wouldn't create an open container of gas to test it. If I remember correctly it is essentiall a rubber diaphragm on the inside, I suspect that one could feel the vacuum with a finger if it is working.
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BroncoJoe19
2006 Alt Fuel Jeep (electric)
'98 windstar 3.8L
'99 Crown Vic 4.6L
'90 Full Size Bronco 5.0L
I'm not a professional mechanic, take my suggestions with a grain of salt, or a cup of coffee 
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