how do i test a vacuum pump and vacuum booster? i have hooked up a vacuum gauge to the the line coming from the pump @ the distribution manifold and with the engine running i have 26.5 inches of vacuum. when i turn the engine off ,i lose all vacuum in about 45 seconds. is this normal? should the vacuum hold for a extended period of time? any info is appreciated. thanks mark p.s. i have an e 350 van ,1985,with a 6.9 diesel.