Hi Lefty
Indeed these decompression valves can develop leaks. One can remove the valve, press the button in and clean the business end in case a piece of carbon got lodged in the valve. I have also seen these valves get bent, how I don't know. If memory serves me correctly, the threads are the same as a 10mm sparkplug thread, so one can use a small sparkplug such as an NGK-CM6 as a block-off plug to eliminate the decompressor valve for testing purposes. A saw of this size will run with no decompressor valve, but it can be a bit of a bearcat to start. Hope this gives you something to work with,
Sky