Ooh, that's a scary one. It sounds like a master cylinder to me. If the booster was failing it ought to require MORE effort. A soft pedal is usually from air in the lines. If you haven't had recent brake work requiring the opening of lines, you shouldn't have any means for air to get in.

That, and the fact that there's no fluid loss (you're certain about that, right?) tells me probably an internal leak in the master cylinder. Replace it, they're not extremely expensive, but a wreck sure is.

The master needs to be bled before it's connected to the lines. I do it on the car, with the two lines disconnected (and capped, just my preference). I use two 12" steel lines that I custom bent for this purpose. I think they cost me $3 each or something. Just mount the master cylinder, install the lines, bent so that they dip below the surface of the fluid in the reservoir, and pump until there's no air bubbles.

Good luck with it!

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