13 inches of vaccuum is not enough, minimum should be 17 inches. The best way to check a booster is with the engine off pump the pedal a few times then holding the brake pedal down start the engine, the brake pedal should go down slightly. There are no springs to either pull the pedal up or down. My advise is first go to basics. Make sure the system is fully bleed and the linnings are adjusted with a slight amount of drag. Next you have to do better than 13 inches of vaccuum. A mild cam with correct ignition timing should give you at least 17 inches, anything less than 17 inches would make the booster ineffective. Also very important is the adjustment of the pushrod between the booster and master cylinder. When adjusted correctly the pushrod should not put any pressure on the master and should not have any gap to the master either. It should have a zero lash.