My Tanks Have a Bad Leak, but theres no sign of it

My trucks been setting in the garage for about a month now. Yesterday I went out there and started working on it. The tanks filled up to 150 PSI and in no time they'd leak down to 80 PSI without me playing with the bags or anything. I let them fill up to 120 and sprayed EVERYTHING with soapy water and nothing bubbled up anywhere. What the heck can this be???