An Astro van has HydraBoost. Meaning that the power steering pump is used for both power steering assist AND power brake assist, as compared to a conventional system which uses engine vacuum for brake assist.
Either your power steering pump went south, or you have a leak in a hydraulic line, causing the power steering pump reservoir to be empty. If its empty fill it up, and identify and replace the leaky component.
I doubt you have a serpentine belt problem since the belt drives the water pump and alternator too ... You would know if you lost the belt, it would overheat and the battery would eventually die out.