Was the twist in The Empire Strikes Back leaked to the public before the movie's release?

Question

There has been an unprecedented amount of leaks available for the latest Star Wars movie, The Force Awakens, including spoilers for all the twists and plot points. This made me think of The Empire Strikes Back, with the famous twist that

Darth Vader is Luke's father

I'm aware that there was a lot of secrecy during production about this twist with only a handful of people knowing about it. But once the film was finalized and especially when it was available for reviewers screenings, the secret would be more wildly known.

In an age before the internet, was The Empire Strikes Back's major twist leaked to the public in any way?

Answer 1

No. Until the first screening, no one knew what was coming other than George Lucas, Mark Hamil, James Earl Jones, and Irving Kirschner. My father told me that when they saw it in theaters, you could hear the wind sucked out of the room. In the bonus disc for the original trilogy, James Earl Jones said that Lucas had pulled him and Hamil into a room, told them that they would know this secret, and that only Irvin (director) knew of what was going on. Jones said "He's lying" (referring to the line), not understanding how they were going to make the story work. He said the line-"Obi Wan killed your father" while on set, then added the real line later.

Star Wars has had very few leaks-thankfully, those on set seem to understand the joy of movies and not knowing what is coming is a big part of the thrill of watching a film.