Actually, in thinking about it more, Champa may be right. It could also be the vacuum distribution block. A better way to check the vacuum pods (all of them) without pulling the dash is to pull the glove box. That will give you access to vacuum distribution box, which is attached to the heater box. Slowly pull off the lines one at a time and attach it to a hand-held vacuum pump. They should hold a vacuum, if not *then* the pod may be busted.

Sorry, missed that critical diagnosis step!