I hook the vac gauge to the valley pan. The relief is built into the pump. Not sure how much truth there is to this but they claim with the relief in a valve cover it could draw dirt into the motor. It can also cause a draft and pull more oil. With the relief built in the pump the motor only see's what it needs to achieve the vac you want to pull. With the relief in the VC, the motor sees all the vac the pump can pull with a cool draft blowing thruogh the motor.