The shop manual has a complete description of the repair of these units. The only thing that would wear out, short of being trashed, is that rubber diaphragm. GM used the same diaphragm in later (circa '70-71) vacuum shifters. I was able to get two new ones from a local GMC truck dealer by perusing the old chevy manual site, finding the part number, and going through my local Chevy dealer. If you do that, they may tell you that the part number is obsolete. Ask them to use their parts locater computer which was succesful in my case. The Chevy dealer sent me across town to the GMC dealer.
I don't have the part number where I am now, but can get it for you later this week when I will be at my shop.
Kent
