Dan, where are you applying the vacuum? Wont the valve guides cause it to leak down? Or are you using it over the valve on the combustion side?
It's a foam covered disc that goes over the combustion chamber. I install everything including the spark plug so there are no leaks. This tool uses compressed air and when you pull the trigger it shoots air out of a small orifice that sits right next to another small orifice. This creates a vacuum that will evacuate anything that will hold a good seal. You can also do individual valves if your application will allow it. I bought it from Souix Tools about 15 years ago.
One time a small piece of dirt fell on to a valve seat during assembly and the tester picked it up right away. I'm sure it would have cleared right away but it will work that well.
You can also vacuum complete cylinders with pistons installed or the complete system with cylinder head mounted. The nice thing about this tool is you don't need to have a running engine to test the condition of the head like you would while doing a compression test. As long as you can turn the engine by hand you will be able to get a reading. Often the readings will be much lower for a complete system due to the larger area being vacuumed and readings of less than 10 inches are common.
I tried using an air pig on a remote test and it didn't work so I think you would need a bottle of compressed air to make it work right if you are not in your shop. I don't use it as much now that have my machine shop assemble my cylinder heads.
