SickSilverado

I drive a 2013 3500hd with a duramax. The rear tank usually has vacuum on it while the front tank has positive pressure when opened. I'd say 90% of the time if I idle somewhere, it'll leave a 3 foot puddle of fuel under the truck. One time I opened the front tank to let the pressure off and I lost 6 gallons. Another time I opened the fuel cap to let off pressure and it spilled fuel for 5 minutes straight. Any ideas on what to look at?