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On Fri 2012-Jul-27 05:39, Roy Witt (1:387/22) wrote to mark lewis:

 ml> i can't say that i've ever known an air horn to work on a vaccum
 ml> system... or air brakes, for that matter... everything i've seen has
 ml> been under pressure...

RW> Yes, air horns require a compressor, but there are also electronic
RW> horns that can wail at 108db that sound like the old air-horn you're
RW> talking about.

I'm not sure this is electronic, large air tank, with a
fitting that can be used to pressurize it.  This is one
reason I wonder if at one time there wasn't air brakes with
hydraulic assist as well as the hydraulic.

There is a regular electric horn though under the hood, but
air horn is atop the cab, at least the "trumpet" part, tank
mounted down low, usual configuration as would be seen in
your usual Freightliner or Kinworth.

Regards,
           Richard
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