<<Here is a mid-80s set of Super Record down-tube shift levers. Following previous instructions, I've gotten them as dry and clean as possible on the inside, and believe me I've tightened the right lever more than finger-tight--even used a small hex wrench to crank some more. But there is still slipping under pressure (standing up on uphills especially).
Is there a trick to make these hold? Is this age and wear or...?>>
Here are two ideas for you, both of which have served me well in the past.
Occasionally, the shift lever bolt -- the thumbscrew that you are trying to tighten -- actually bottoms out in the shift boss, without generating the desired friction. Solution: shine a flashlight in the boss, see if there's gradue (that's a technical term) in there; and/or, grind off maybe a mm or two of the end of the bolt. I had to do that more than once.
I also used to schmear (I rode with a Jewish cycling team, Velo Loksh -- and if anyone gets THAT joke I'll be very impressed -- so we spoke lot of Yiddish) a little sew-up glue onto both sides of the shifter washers. Let the glue dry, reassemble, et voila! Rubber cement, barge cement, etc. could be substitued for the gutta.
Happy trails,
Peter Bridge
Not far from Denver Intermational Airport