The "block check" test works well AFAIK but we have had some people say it didn't work for them and I don't know if something has changed to make it not as reliable as it used to be back in the day. Anyway follow the instructions and treat it like a smoke test, If it shows a problem there is a problem, but if doesn't then doesn't mean there isn't one.
You might try driving the car for a while through a number of heat/cool cycles without taking the cap off so that the gases might have a chance to accumulate a bit more before testing. Then follow the instructions. Get the tester on quick after removing the cap. I remember we even had someone from the company that makes the test kit chime in on a thread in support of it!
