It is normal for the air system to leak down when parked for a few days, If all the air hoses, air valves, relays, and everything in the air system were perfect it would not leak but that just doesn't happen to often, usually the air will leak from a relay or valve that has a little gum or dirt in it causing the seals to leak. next time you park it will be a different valve or something. The allowable leakdown rate according to the DOT is 4 psi in the front and 4 psi in the rear in two minutes which figures out to complete leakage down to zero in one hour, check this web page for a better explanation, D.O.T. Standard Allowable Leakdown
Bill And Jeanne Anglin, Wasilla, Alaska.
1999 Dutch Star 3884, Freightliner, Cat 300, Jeep Liberty Toad.