You left out "block check". It is a process where you draw the air in the top of the expansion tank through a special fluid that will change color in the prescence of CO. Block Check is the brand name, you know like Xerox or ChannelLocks. The process is pretty good but in the same respect the leak must be fairly substantial. It often times will not pick up a intermittant leak, you know the kind that is there when cold but goes away as everything heats up. I purchased a kit off of the Matco truck years ago and have not been able to find it since. This kit worked in the same manner except it uses a different test fluid and it tests for the prescence of CO2 and this system was pretty much down to a gnat's ass when it came to detecting leaking head gaskets. Keep in mind that the exhaust sniffer and the Block Check type leak checkers all work on the premise that it is leaking exhaust gas into the cooling system and for the most part most leaking head gaskets will do this, but not all. That being said I do not think that any of the aforemention methods are 100% foolproof. The two I do like the best, and I've used them all, are the exhaust sniffer at the bottle or the CO2 detector kit that I got off the Matco truck.

Mark