Eng. 12 requested to assist Eng. 62 with a large amount of fuel on the ground. Upon inspection of the truck it was found that the pick up had a secondary fuel tank in the bed. Along with that there was a secondary fuel pump which was causing the fuel to continue to spill. Fuel continued to spill with the truck itself being off until access was gained to the secondary fuel pump that was malfunctioning. After access was made the secondary fuel pump was shut off and the fluid stopped spilling out of the truck. ENG 12 was released while ENG 62 remained on scene until a clean up crew arrived on scene.