Brakes are strange

Got back from San Diego yesterday, and we did take the convertible. But while down there it started doing something strange with the brakes. Need some feedback.

Pulling up to a stop, the brakes start out fine. As the pedal goes down, the power boost gradually drops off, until there's none left. It doesn't lope, as if there's a vacuum leak, but there's no brake boost. Pumping the brakes does nothing, and it does it every time I brake. I've seen brake boosters go bad before, but they didn't act like this. It didn't get better or worse during the trip. I'd lean toward a vacuum leak, but there's no sign of it in the engine. I don't want to just start throwing parts at it - I can't afford that.

I've never had a brake booster check valve go bad - do they feel like this?