I'll see if I can't answer everyone's posts here...

@RapidRobert: Right, I'm using that Dorman(sp?) brake bleeder kit. Plastic hoses with a assortment of plastic fittings. And yes now that you mention it, those fittings don't fit too well, so they may very well be sucking air somehow- although I'm not finding any seepage from around the plastic fittings. But I'll check on the rear plastic fitting and try to get a better seal on it...

@John_Kunkel: It's definitely air escaping from the port. Specifically, the port furthest back on the MC. Actually there are 2 ports back there, one in front of the other, and I've seen air bubbles coming from both. The port in the front reservoir isn't bubbling, though. And you might be right about the booster making the sucking sound- I'll see if I can't pin down more precisely where the sound is coming from to be sure.

@Pynzo- I'll give that a try (especially if I can't get the plastic fittings to seal) if I can find my set of old lines (I replaced the brake line some years ago- I THINK I have the old lines somewhere)

Might not get to doing this 'till the weekend, but I'll give it a shot and report back in. Thanks for the tips!

David