An employee was delivering a load of peanut oil in a food grade liquid oil bulk tank. While he was disconnecting the hose from the truck's tank, he reversed the pump flow to remove the excess oil from the hose. The hose plunger was not inserted at the time. When the employee attempted to re-insert the hose plunger, the suction of the pump caught his left index finger between the pump and the plunger, crushing his fingertip, which had to be surgically amputated.