Brake problems - pedal dropped

When I was out driving yesterday, the brake pedal suddenly sank to the floor. When I pressed it again it was fine. Then after a while it went down again, only to stiffen up at the second try.

I haven't had the time to look into it, but does anyone have an idea what it is? I'll probably replace the brake fluid and bleed the system just for the heck of it, but I hope I won't need to repair calipers or anything. Btw, the braking power is OK when the pedal is behaving.