Brake pedal stays down?

When I step on the pedal in my Aspen the pedal feels normal, but the pedal stays down after a take my foot off, and the brakes stay applied. I can pull the pedal up, and the brakes release. I'm not sure where to start. Pedal linkage? Master cylinder? Booster(seems doubtful)? Collapsed hose?
I've never encountered this before. Anybody have any suggestions where to start? Anybody have this happen to them?

Oh yeah, the car is a 79 Aspen, 318/904 with power brakes. Totally stock 79,000km car (about 50,000 miles for you Americans) that doesn't seem to have had, or needed much work over the years. But, I don't actually have any service history...