Brake issues

My Duster has disc front and drum rear brakes it has been driven 7000 miles and worked
fine all of a sudden the brake peddle goes to the floor when you sit and hold the brakes on in gear I have adjusted the rear brakes and they are snug there are no leaks anywhere I can find and if you pump the brakes when the car is not running along at 50 or so it seems to hold pressure however when you are running it seems to work until you slow down and are idling in gear. I have replaced the vacuum hose to the booster didn't seem to make any difference. The vac booster was rebuilt 3 years ago. Does that sound like a problem with the master cylinder or with the vacuum booster.