Vacuum testing and what hose to put it on

You will need to connect your vacuum to a "full" manifold vacuum line on your 1999 GMC Yukon. There are two types of vacuum ports available, ported vacuum and full vacuum. Full manifold vacuum means the there is vacuum on the line at idle. Ported vacuum means there is only vacuum when the throttle valve is opened slightly. The PCV and Break booster hoses are both full vacuum. Any vacuum hose that has vacuum at idle is full manifold vacuum, that is what you need. There may be some smaller vacuum lines that you could use other than the PCV and Brake Booster. When you connect your vacuum gauge be sure to properly "tee" into whatever vacuum line you are going to use.