Are you sure it's an air lock and not a valve turned down? If it's an air lock I doubt draining the system down will do any good and may make the situation even worse. What you probably have is air trapped in a high spot in the hot water pipe system. The solution is normally to force mains pressure water up the hot water pipes from the tap which no longer has water running out, not always so easy with modern tap design.  Ideally you could do with a vent fitting in the high spot where the air is trapped but you may be able to put a strong plastic bag over a mixer tap, seal it as best you can ( it does need a good seal) and turn both taps on. It normally only takes a few seconds of water running back to clear the air. Good luck!