The release button usually isn't the problem. It's the Schrader (tire) valve in the end of the probe. A tiny bit of carbon or other debris on its seat will let the pressure leak down or at least give an inaccurate reading.
Cylinder leakdown testing is MUCH more effective than a compression test, and it doesn't take extremely sophisticated equipment. All you need is an air source and a pressure regulator, plus a home-brewed fitting to replace a spark plug. Set the regulator to 100 PSI with no airflow, then plumb the air to a cylinder at TDC with both valves closed and the crankshaft locked. Whatever pressure remains indicates the percentage of leakage- - - - -for instance, 90 PSI indicates a 10% leakdown rate. Listening for airflow at the carburetor, tailpipe and crankcase tells where the pressure is going. Bubbles in the radiator show head gasket leaks.
Jerry
