I'm new here, but a little info in brake systems, the brake booster operates off of manifold vacuum, that being said, it's a "one" way line, there should be a small check valve in the line coming from the manifold, or attached directly to the booster itself. A good way to check for vacuum leaks around the manifold is with a can of WD-40, get the car running, and then spray it around the manifold base and anywhere else you may suspect a leak, you will notice the change in engine rpm very quickly. A fast and dirty way of tracking vacuum leaks. Hope this helps.