Purney
I used a rubber hammer to tap the top of the valve, with the rocker removed while spraying WD-40 on it and after a while the WD-40 washed the varnish and the minor surface rust away. You can tell when the valve is opening and closing properly by the popping noise it will make. It might stick open for the first couple ( or more ) taps. If it sticks
just get a small block of wood and a screw driver and pry it back up. this is a bit of a non proffesional way to loosen up sticky valves but it has worked for me in the past on a couple of old tractors that sat for a long time, just make sure you tap it square on the top and not to hard, you will be able to tell how hard to hit it after a couple of lite taps.
good luck
DS