81 C10 Diesel leak at sending unit

[brandon.pasley]

Bought my son an 81 C10 5.7 Diesel and am having an issue with diesel leaking from the sending unit. The sending unit has three lines; one's a supply, one's a vent, and one's a return. The vent has a plastic cap on it and for some reason diesel leaks from this. Nothing seems to be affecting driving the vehicle but it will leak for a while then stop. I'm wondering if this is an emergency vent to prevent pressure build up in the tank. Any help with solving this would be greatly appreciated.