[elhigh]

When I installed my vac gauge I hooked into the vacuum line feeding the brake booster. Since that's the one place you know you'll want all the vacuum you can get, you know that's the one place it won't be metered or otherwise interrupted anywhere. A couple of vac sources are like that.

Do, of course, make sure you aren't hooking in "downwind" of the check valve that holds the vacuum in the booster. Your vac gauge will go right up and stay there even when you turn the engine off. Weird and obvious when you think about it.

They recommend against it citing safety reasons, but mine has been on for a few years now and has presented no issues whatsoever. In fact the gauge has been less reliable than the plumbing, but that's another story.