I believe it is to run a second vacuum pump for trailer brakes, the type that Midnight Rider perfers. The trailers he pulls use vaccum to actuate the TRAILER brakes, rather than electromagnets and electricity that we are most familiar with. He says they work very well. I have not used this type of brakes, and have no experience. Reminds me of a semi using air for the brakes. The tractor supplys the air, or in this case the vacuum to the trailer. I would not want to run the trailer and truck brakes of the same pump, since if you loose the pump, not only do you loose the assist on the truck, you would loose all brakes on the trailer. With 2 pumps, at least one pump should still work, and one set of brakes would work. I would rather stop the truck and trailer with no power assist on the truck and working trailer brakes, than manual brakes on the truck and no brakes on the trailer....