Alright how about this, pump it up stone cold to 15 & let it set overnight & if it drops to say 10 then we have lost 5 lbs of psi somewhere & now that might be 5lbs of air at the top of the system or coolant in the rest of the system or a combo of both but we ARE losing coolant & tho you and me neither want it to be a HG issue, that 5 psi loss (& some if not all is coolant cuz we are losing it) then it has to be going somewhere