Air operated flush sticking valve?

My toilet bowl has water running unto it constantly. On lifting the cistern cover I can see that the air pump arm and the plunger that releases the water are stuck together in the up position. If I nudge them they release and work normally. I have removed and checked them, they are a loose fit, so should not stick. I have tried lubricating the fork with fairy liquid and Vaseline, still sticking. Air valve is working fine and plungers on both move freely, just sticking together at the top of their travel. Not sure what to try next? any ideas?