RW> ml> i can't say that i've ever known an air horn to work on a vaccum   
   RW> ml> system... or air brakes, for that matter... everything i've seen has   
   RW> ml> been under pressure...   
      
   RW>RW> Yes, air horns require a compressor, but there are also electronic   
   RW>RW> horns that can wail at 108db that sound like the old air-horn you're   
   RW>RW> talking about.   
      
   RW>I'm not sure this is electronic, large air tank, with a   
   RW>fitting that can be used to pressurize it.  This is one   
   RW>reason I wonder if at one time there wasn't air brakes with   
   RW>hydraulic assist as well as the hydraulic.   
      
   What Mark was talking about is the Vaccum powered air compressors. They   
   have a very long pressure recovery time so use with an air horn would be   
   limited. They work great for Air bag assist on suspensions though.   
      
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