The button is attached to a small piston/cylinder. They can wear and become faulty/leak. If blowing down the tube is working then this is the fault. But this is better/cheaper than the fault being the other end. You WILL need a new piston/cylinder which is best purchased with a new button.
Search Internet/eBay for "Air button" in home and garden category. They seem to be about £25. Hope it helps.
J
April 2011