I have a 2014 F250 Crew Cab 6.7L. I added a Aux. fuel tank to bed of truck. I purchased tank from RDS ( 45 gallon ). I use gravity to feed the factory tank. I installed the gravity kit that RDS supplies when I purchased the tank. I installed everything correctly in April of 2015 and have had only one issue until today. The one issue I did have was fuel leaking out of brass vent on top of aux. tank when tank was full. I ran a plastic hose out of vent , looped it and tucked inside the bed of truck, which works great.No problems with that since. That beside the point, the issue I came across today was fuel leaking around factory tank. It looks like it is coming from the top. I did fill the Aux. tank and factory tank two days ago. On the aux. tank I did install a shut off valve, but I usually leave it open and have had no issues until today. I checked all the clamps and fittings I installed when I put the aux. tank in and nothing is leaking. I shut the valve off on the aux. tank and I think it stopped leaking but not totally sure. I wiped everything down the best I could and will check it in the morning. Any thoughts on what would be leaking on top of tank? Too much pressure from Aux. tank causing a seal to leak maybe? Any help would be appreciated......

 

Did you ever figure out what was going on with your leak? Woke up to the same issue today, only difference is my truck is a 2012.

 

RDS has had problems like this, they recommend you run the vent hose on the OEM tank up into the bed rail as high as possible, preferably, to get it higher or as high as the AUX tank vent. They still suggest to close the AUX tank valve when your OEM tank is full to prevent overflow. I have read where others have replaced the manual valve on the aux tank with a switched electric valve they can turn on and off from inside the cab. Good luck, let us know what you find or decide to do.