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I have a 81 and my drivers side sunvisor will never stay up.
I have replaced the sunvisor bushings many times, like 3 or 4 times. I even went as far as replacing the entire unit. But after a month of hot weather the bushings get soft and don't keep the grip and no longer keep the visor up.
I know it kinda sounds like something Bubba would do...but what I did..and it works..is wrap a little teflon tape around the elbow between the two bushings.
I don't know if a '79 is similar, but mine has a plastic bushing that has cracked, making the visor not stay up. My bubba fix was to carefully let some thin superglue wick down in around the bushing and it's now perfect!