Question: How did Luke know that removing the mask would make Vader die?
Piemanmoo
13th Jun 2005
Star Wars: Episode VI - Return of the Jedi (1983)
Answer: It's almost certainly known - through Rebel intelligence gathering if nothing else - that Vader's suit functions as a life support system and the various details that go along with that.
Chosen answer: Obi-Wan told luke how his father was more machine than man now, and Luke knew Vader had a life support system, so he concluded that he would die without his mask, especialy given the weak state he was in.
Piemanmoo