my wife's complained that the brakes in her '79 300D started acting up last night. I drove it around for about 30min and it seemed fine....but then I drove it about an hour later and the brakes were rock hard. Really difficult to bring the car to a stop.

After reading some other posts, I see that this is probably either a vacuum leak or the brake booster. Since I'm not having any other vacuum assumptions (engine run-on, locks not working, etc) shall I assume it is the brake booster? If not, what sort of diagnostic should I be doing?

And I wonder where I can get a brake booster on a Saturday?? -Harv