S&W 59
I have had a nickel-finish model 59, also, since 1979. The take down lever has always been difficult. You can download the manual online and it will give you some pictures. First, and for safety sake, I make sure it is unloaded. Now that we got that out of the way, pull and hold the slide back to where the detent on the left side of the slide is over the lever. Then, I press the protruding part of the right side of the take down lever against a small piece of wood. I grab the left side of the lever as it gets pushed up. When the lever is out completely I move the slide forward and make sure the spring does not rocket off into space.
Hope this helps.