Well i doubt your at 0 elevation so we will say you had a DA of 3500, that would put your absolute atmospheric pressure at 12.93, so redo it and plug that in 31.04 minus 12.93 = 18.11PSI

If you can tell me your Density altitude at the time you measured i can give you a pretty close number of psi your running, i dont think you have a leak that sounds about right.