They will bleed off some small amount when the truck sits. Mine would do that when I drove OTR. The shifting of a trialer in the wind, the poressure of the weight on the bags, the slow drain from the exit valves, etc, will usually run the air down. You should drain the tanks every couple of days using the water drains anyway, even if you have an air dryer, to keep moisture from accumulating in the system. It will really screw it up, and repairs are a major pain and expense.
You may have a slow leak too, as I don't think brake use of that type when the truck is running should cause the low pressure warning to come on.
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