Greg,
If you have a choice, find a tester that has screw-in plug adapters. They make the test much easier and more accurate.
If the plug location is such that you can't get a good strong shot at it, it isn't uncommon for the pressure to blow the little rubber doohickey right off, or push it up enough to give a inaccurate reading. If you don't know this you might think you have a bum cylinder.
Stuart
