Brake Pedal drops to floor when truck starts..

[Jefe]

(Drum brakes all the way around) This thing has me stumped...need some serious help. The pedal can be pumped up when the motor is not running and stays really stiff, doesnt fade, not mushy, and doesn't drop. When truck is started it drops almost to the floor. New parts less than a week old include, power brake booster,new power brake booster check valve, master cylinder, front two wheel cylinders, new shoes and drums on all four wheels, new front rubber brake lines. I'm inclined to think it is a vacume booster problem because it only goes down when motor started. It is connected to the back of the carb. and is drawing about 13-15 inches of vacum at idle. Any ideas are completely welcome!! Thanks guys!!


Douglas.