Could a val deny a request by the king?

Well a val is the ruler of his own estate. I don't think he directly serves the king, and thus if he felt as though he were about to operate good enough on his own without the aid of the king i see no reason why he would need to obey. Since the vals' armies were probably the strong arm of the king, it's not like the king can just order them to sacrifice themselves right? I suggest reading into feudalism more to understand the answer, or waiting for someone who knows the answer on here xD