Hi QSR,
Just a thought, perhaps your decompressor has failed, if you remove it and suck on the valve it should stay shut. The only other thing I would suggest is to replace all the o rings, they are cheap and easy to do.
Also check under the mower to see if something like wire has wrapped around the output shaft and buggered the seal.
I haven't tested the decompressor but had a few lying around and tried them with no luck too. The only seal I haven't changed is the starter one.
