How about using a vacuum operated windshield wiper motor instead? They
have a lot more power than a vacuum advance would have. This "old
stuff" isn't gone, you just have to know where to look. In Minneapolis
Mn there's a place called "Little Dearborn" where you can get almost
anything for old Fords at least. I didn't say "cheap", though.
I'd also try an outfit like Bimba who makes air cylinders. They make
em pretty small and the difference between air pressure and vacuum isn't
all that great.
Also, there are companies that make vacuum sensing switches. We used
to use several open bellows types. There are your bellows.

Pete Stanaitis
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Chet wrote:
Hi Guys!
I run a 1/8 scale live steam locomotive.
While testing the vacuum brakes on my tender, I blew out the diaphrams
that activate its brakes.
The original unit was used as a vacuum advance for old-time carburetors
and is now an endangered species.
Anybody have success making their own from scratch?
If so, would you be willing to share the process?
Thanks in advance,
Chet (engineer of ANCR Mogul #9401)