81 C10 Diesel leak at sending unit
 

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.