Fixed it! The check valve on the bottom of the bladder wasn't going deep enough into the recepticle on the base of the unit; the valve wasn't opening, and the pump was sucking from a dead end. (It WOULD pull soap from the small tank, but not the main bladder reservoir.) I cut a 3/4" piece of a disposable ball-point pen cap, to ride over the rod in the center of the recepticle. Long enough to push the valve open, but short enough that the tank assembly sat down on the base. If the rod is too long, it will push the whole tank up, and break the vacuum seal at the forward edge. Evidently this thing was designed to a tight tolerance, and just a little aging/distortion of the plastic reservoir base was just enough to keep the valve from being pushed open!
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