Leakdown tester for lawnmower

I just bought a leakdown tester for $35 on eBay, along the lines of the HF model. Just figured I'd give it a try to see if it's something I'll get a lot of use out of before springing for a better quality one, like a Snap On. Anyway, the part that screws into the spark plug hole has a schrader valve on it. It is therefore not letting any air into the cylinder.
When I press on the schrader valve, it leaks, and registers the leak on the gauge. So it appears to be functional otherwise. Is...

Leakdown tester for lawnmower