I had the same problem and you are right, it was one of the check valves located close to the engine side of the firewall on the drivers side. They are round, like 3/4 " plastic valves connected to the vaccum line. To test them just take them out (Remember the positition for reassembly) and blow with your mouth on one end, air should pass throught only one end. If air passes throught both ends, replace it. Good luck.