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Spongeous-- I like that word!
I'm thinking that there's something wrong with the booster if it's returning with no vac on it. There has to be some sort of valving assembly somewhere on the booster. The booster has vac on the "wrong" side (i.e. the side towards which the actuating rod moves) from a normal pneumatic system. It uses atmospheric pressure acting on the other side of the diaphragm. The brake booster is directly attatched to the master cylinder rod and the pedal, so it moves with the pedal. If the vacuum isn't releasing, you'll be getting boost when you shouldn't and the pedal will feel soft. This could also result in it not returning. Similar symptoms could be present if the booster isn't releasing the air that comes in to fill the void made by the diaphragm.
You've already answered a very big question by trying the brakes with no vac on the booster. Since the pedal returns when the brakes are straight hydraulic, we know that there isn't a blockage in the brake lines preventing the fluid from returning. We know that the spongeous (new favorite word) feeling isn't from air in the system, because it goes away without the booster. So we've narrowed it down to the brake booster and related vacuum harness. Try getting a vac gauge on the booster line just to make double sure everything is good, and see if you can find the air intake for the booster. It should be inside the passenger compartment somewhere, immediately where the pedal rod goes in to the booster, since all booster designs I've seen draw air from that location. Make sure this is clean. Not sure if the booster is serviceable or not. I imagine no, but then again, the alternator "is" and you can do brushes on those.
 
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