The rubber parts in the vacuum shift cylinder would get sticky and refuse to move smoothly. The fix was simple- - - -run a 6 ounce can of Casite Tune-Up through the shift cylinder by letting the engine vacuum at the cylinder vent suck the liquid through it. Then the same solvent would serve double duty by cleaning carbon off the valve guides. The same procedure also worked well on vacuum windshield wiper motors, BTW!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!