Hi John,
Here is the best I can explain. This was taken from a source on the net I found.
"Most air brake equipped vehicles on the road today are
using a dual air brake system. The system has been
developed to accommodate a mechanically secured
parking brake that can be applied in the event of
service brake failure. It also accommodates the need
for a modulated braking system should either one of
the two systems fail. It is actually two brake systems in
one, with more reservoir capacity resulting in a much
safer system. At first glance, the dual system might
seem complicated, but if you understand the basic air
brake system described so far, and if the dual system
is separated into basic functions, it becomes
quite simple.
As its name suggests, the dual system is two
systems or circuits in one. There are different
ways of separating the two parts of the system.
On a two–axle vehicle, one circuit operates the rear
axle and the other circuit operates the front axle.
If one circuit has a failure, the other circuit is
isolated and will continue to operate."
What you are referring to is like the emergency valve that Lee was discussing. I like what you call it better "Dead Man" switch. I think we are talking about the same thing, just your is set up different. Do you have a schematic showing the plumbing?
Paul
