Its been a while since I read all the engagement literature but my thinking was that if he was sitting idling since your idle drops off after the truck warms up say from 1500-1100 cold to 500 warm depending on the outside temp that the vacuum would drop off. So if there was a leak it would be enough to cause the system not to engauge at idle. Then again I could be totally off in left field.

1997 T-100 100k and counting Green
5spd 4x4 Ext Cab isr mod Aisin Manual hubs