I don,t pretend to be a qualified mechanic, but when I ran trucks years ago, if something like this happened, the first thing I would do is take off the valve and clean it thoroughly, 9 times out of 10 that cured it, it was just a piece of dirt holding the valve open. If it doesn,t work, buy new valve (or get one from a truck dismantler and fit, it looks like a bolt on job. (Make sure you have the correct valve though! some may look the same but operate at different pressures.) Good Luck.