How do you know if you have a vacuum leak on a 99 jimmy?

Sometimes it can be as simple as looking for a hose to that has come off or a broken fitting. This can be done by isolating different areas in the system.

If the above fails you need to measure the vacuum with a vacuum gauge. While the truck is running or how fast the vacuum dissipates when the engine is shut down. Then you use a MityVac (small hand held vac pump) to measure the different areas where the vacuum is supplied and fix the leak when you find it.