Leaky 2-way check valve between the primary and secondary tank possibly.
Air up truck to cutout, shut it off. Open the drain on the primary tank and drain it down to zero. Check gauges. If both read zero, check valve is likely shot. If primary is zero and secondary is still full, build air back up and repeat but drain the secondary tank this time. Secondary gauge should read zero and primary should stay full. If both are empty, likely the check valve.