Carl, you once you have acheived 20"hg of vacuum in the tank, turn the pump off. You will not lose very much vacuum unless you let air into the system. What diameter and length of hose are you using?
Check out my company www.sageoilvac.com. Our units are made for changing oil, but we have customers that use our units for gathering waste oil from farmers. We usually acheive 20"-24"hg of vacuum on our tanks.
Too bad Southwest Implement doesn't pick up the waste oil before it gets cold. Its hard to suck molassis through straw.

Phil

Never hold your farts in. They travel up your spine and into your brain...that is where crappy ideas come from.