Why mess with your trucks vac system, just go to the junk yard and look for an old caddy. It has a kickas vac pump, drivers side front, under the battery box. It is 12 volt and will pull a 45 gallon blatter tank down to 20 in lbs in about 4-5 minutes.
I mounted mine under the bed on a toggle switch, and ran a piece of air line hose through the drain hole in the bed... . Instant vac when you want it, works real slick. Got mine for 5 bucks.
Dirk