Did you check the brake booster to see if it is leaking down after the engine is shut off? You do this by putting your vacuum gauge on the vacuum line T closest to the booster while capping off the other vacuum line. Start the engine and you should see at least 20" of vacuum. Shut the engine off and the gauge should continue to read at least 20" for quite a while (several days). The other check is to shut the engine off and you should be able to pump the brakes 3 or 4 times and not have a hard pedal which indicates you still have vacuum assist. While pumping the brakes you should see the vacuum gauge go down.