Joe is likely the guy that's most knowledgeable about this, but both my GL's that had the air shocks ('84 1200I and '91 1500) did tend to bleed a little, but not to the degree you seem to have. My Interstate would slowly loose pressure over long periods of sitting, like over the winter, and at full pressure and full loads for trips with the Mrs, I would have to touch the shocks up a couple times over the week or two that we were out, and we're only talking a few psi here. My 1500 did bleed, but not near as much and maybe I was more dilligent with it as I had the on-board compressor, so I didn't need the service station air.