The vacuum motor is probably dried out. You can not get parts for these but the can be sent out for a rebuild. Try taking the top off very carefully and cleaning the insides, adding some soft grease, and a little oil to freshen up the leather paddle. You may find it starts working pretty good. 17" is plenty for them to work. A little rain-x or wax on the glass helps.

Check with the Filling Station or any of the venders on this site for parts. See links above.

Joe

1937 Chevrolet Inline-powered Streetrod