One thing to bear in mind is that a really tiny leak in the priming pump intake valve will cause exactly what you are experiencing. Taking out the moving parts of the priming pump (most likely in the cavity right behind the primer bulb) and carefully cleaning them might do the trick. Remember, you are trying to get a liquid pump to work well pumping air: it doesn't take much of a leak to stop it in its tracks. Be gentle - taking Chinese plastic pumps apart and reassembling them successfully is a job for a watchmaker with a sense of humour.
