Locating Vacuum Leaks

The following write up was done by Chris (AKA PKRWUD) from MustangWorks. It was a response to a post about locating a vacuum leak. The write up was so good I had to post it for everyone to see. This is the best write up I have seen on this subject and it is all you will need to locate any vacuum leak.

A vacuum leak will have little or no effect above idle. The first thing I ask
my customers and friends who suspect they have a vacuum leak is this:
If you were to enter a panic stop situation, would the car stall ? If the
answer is yes, you've got a vacuum leak. You can hook up a vacuum
gauge and note the numbers. A stock engine in good shape will be
around 17" to 20". A modified engine with a mild cam will be 15" to 17".
A race cam will bring it down to 12" to 15". These are the numbers for
an engine with out a vacuum leak. If your motor is otherwise healthy,
and your timing has been confirmed to be correct, a reading of about 5"
lower than these numbers will indicate a vacuum leak. To locate it, first
check all your vacuum lines for cracks. If they all appear to be intact,
start up the engine and spray WD-40 around all the intake manifold
mating surfaces. What you are watching for is a sudden rise in rpm. If
this happens, you've located your vacuum leak. The last probable
location for a vacuum leak is internal. This is especially likely if you have
recently changed your intake manifold. Fortunately, there is a quick and
easy way to test for this;

1) Hook up a vacuum gauge to the intake manifold
2) Make sure the throttle plates are fully closed
3) Plug up the PVC valve opening
4) Disconnect and plug up any vacuum hoses attached directly to the
    intake manifold
5) Disconnect the battery feed to the ignition coil to prevent the engine
    from starting
6) Crank the engine and observe the vacuum gauge. From 3" to 7" of
    vacuum (at a minimum of 250 cranking rpm) is normal. A defective
    intake gasket will give, in most cases, a zero to 1" vacuum reading.