Okay, I just talked to Cummins Fleetguard Tech Support and they told me the chart we are looking at is old (2006) and that diameter on the AH8505 is wrong. It is 13.5".
I also told him what we are working on and he said 1420 cfm is plenty for an ISX and the problem is most likely the filter minder. I should see if I can find someone to check it or just replace it and see if that solves the problem.
He also said the canisters with removable filter are better than the disposable cans, and if there is still a problem they could spec a canister and filter that would improve air flow.
How is a filter minder tested anyway, vacuum pump with a gauge I suppose.