Need advice on brake issue please!

I have thought about that possibility. but when i pull the check valve off the booster after it has been running you can hear air rushing back into the booster. I know it is holding vacuum. How else can i test the booster. I even ran a long vacuum line from an 05 chevy silverado to my dart to see if i could get any assist while i was just parked. It didnt feel any different then.