milk the bull

old-fashioned To engage in an activity or enterprise that has no chance of succeeding; to do something pointless or futile. (A bull is a male cow and cannot be milked.) Everyone told us we were milking the bull when we set up our online store during the infancy of the Internet, but now we're one of the most successful companies in the world. I don't know why you bother arguing with him. You're just milking the bull if you think you're going to change his mind.

milk the bull (or ram)

engage in an enterprise doomed to failure.