I'm leaning towards a booster problem too. But a restricted vacuum line would not be out of the question. Gives kind of like that feeling. Starts to pull down but then loose ssome of the assist during pedal travel. The vacuum valve I have on the booster has a small hose port that I plugged off. I could hook up a gauge to that and check vacuum at the booster.
I'll do that, check the vacuum source at the booster while operating the brakes, and check the disk calipers. The booster was off the car for all of those 7yrs it sat in my driveway (think it was in the trunk IIRC) maybe some rust built up in it causing it not to function properly. Everything had a little rust when I started.