On Tue, 26 Aug 2008 17:29:50 -0400, "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)

Haven't built one, but a distributor vacuum advance was expected to
move a few grams of linkage, not transmit enough force to (in concert
with two dozen others) stop several hundred pounds of train and
riders.

You might have to come up with your own design - make two shell
halves that bolt together with 6/32 screws to roughly duplicate the
shell of your vacuum advance, get some fabric reinforced neoprene
gasket material - think fuel pump diaphragm, and two fender washers
sandwiched in the middle on a rod nut for connecting an actuating arm.

Go take apart an old mechanical fuel pump for ideas on how to build
the working section - they transmitted the levels of force you need.
Or look at a truck air brake actuator for ideas. Scale down.

Or see if you can find a miniature air cylinder that is the right
size and shape for the job, and you can just buy them and drop them in
instead of reinventing the wheel. Clippard or Cincinnati or Bimba...

On that note, you can get a lot more power from air than vacuum,
even under "Direct Air" - or duplicate the full WABCO Relay system in
scale, and if the train brake pipe breaks everything stops.

-- Bruce --