For those watching -So far still holding vacuum.
Bike patch kit was $3.99, enough patches in there to thoroughly repair at least 4 diaphragms. Seems it's generally the top ones, exposed to grit and gunk over the years that need it. So, for I'm guessing 8 bucks, you could potentially avoid buying 8 new diaphragms.

this is encouraging. thanks for the followup.

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'77 240D, 504H, OM617.952, etc.