It's accepted almost as a fact that McCain acted heroically in refusing early release and while he acted properly he would have faced (at least) a court martial had he accepted that. Under the provisions of Executive Order 10631 of Aug. 17, 1955 all military members were to adhere to the following:
Code of Conduct
for
Members of the United States Armed Forces
I
I am an American, fighting in the forces which guard my country and our way of life. I am prepared to give my life in their defense.

II
I will never surrender of my own free will. If in command, I will never surrender the members of my command while they still have the means to resist.

III
If I am captured I will continue to resist by all means available. I will make every effort to escape and aid others to escape. I will accept neither parole nor special favors from the enemy.

IV
If I become a prisoner of war, I will keep faith with my fellow prisoners. I will give no information or take part in any action which might be harmful to my comrades. If I am senior, I will take command. If not, I will obey the lawful orders of those appointed over me and will back them up in every way.

V
When questioned, should I become a prisoner of war, I am required to give name, rank, service number and date of birth. I will evade answering further questions to the utmost of my ability. I will make no oral or written statements disloyal to my country and its allies or harmful to their cause.

VI
I will never forget that I am an American, fighting for freedom, responsible for my actions, and dedicated to the principles which made my country free. I will trust in my God and in the United States of America.

Note article 3. That McCain would have suffered grave consequences is especially true as the son and grandson of Navy Admirals because most Americans would be outraged.

McCain acted properly, that is to say honorably but it was no special act of valor.

Edited to add: And of course Trump did not act honorably...