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?