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Tom,
I would expect either a mechanical linkage to a lever which
the driver pulls or pushes as in an the 18 wheelers of long ago, or a switch
which will allow me to close the butterfly valve ONLY WHEN I CHOOSE to
apply exhaust brakes.
I can quickly think of many situations in which I would lift
my foot from the tow vehicle throttle but would not want a mechanical device to
assume that I wish to have brakes applied for me.
Harvey
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