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Hello, I’ll be doing a vacuum test for my car, it’s good and runs pretty well for the 150k miles, but my question is where is the best line for testing the vacuum system? I’ve heard brake booster hose…

 
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Brake booster line for sure. I used it as a vacuum manifold when I was turbo'd with my EM2. It does have a check valve in there, so there's some food for thought depending on if you're either cutting it to install a permanent T fitting or just want to disconnect something to test for vacuum loss as a one time thing. If you're installing a T, make sure it's upstream/engine side of the check valve. The only real good way to find it is by squeezing the hose

edit: pics from 17 years ago when I tapped the wrong side of the check valve for my vacuum manifold are in this thread: https://www.civicforums.com/forums/7...lem-fixed.html

the check valve is about an inch to the left of the T fitting I installed. Anywhere on the engine side of the white clip would be a good place to tap

 
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Thanks for helping me out... Yes, the check valve in inside the hose. I will do a test not cutting nor installing something, but I think the valve will not affect the readings, since is a one-way valve.