JMH,
You can't just call a Chevy dealer. They have no way to look it up. You need the part number and then ask for them to use their parts locator computer. Then they will just tell you whether they have a listing, but won't know what kind of part it is. I called Chapman Chevy in Tempe, Az (480.838.1234 and ask for the parts dept.). They have helped me twice but didn't sell me the part, just gave me the number to call where their computer indicated the place had what I was looking for.
Using the
http://chevy.oldcarmanualproject.com site I believe I found the part number 3847058. It is group number 5.635. Look on pg. 504 in the 1929-1957 Master Parts List and Accessories. If you can wait, next week I will be up at my shop, 120 miles away from here, and will check to see if I still have the box my new diaphragm came in. If I do it will have the part number on it (maybe).
If this doesn't work let me know as I have a contact where you can call and see if they, by chance, have the part.
Since your diaphragm is toast, you should have a major vacuum leak, regardless of whether you selector is in high or low range. If your truck is running, you need to block off both hoses at the back end until you solve your problem.
Isn't this fun? Working on these old beasts can be a challenge but you will get it done. I think it will be easier to find that diaphragm or even change out the rear axle, rather than engineer a mechanical selector solution.
Kent
