From: James Shanks (n1vbn@bit-net.com)
Date: Thu Dec 25 2003 - 20:07:03 PST
Hi Greg,
The answer is simple, the trailer breaks away from the towing
truck it was attached to and rips both hoses loose. The service hose which
transfers the signal from your brake pedal to apply the brakes on the
trailer and rips the emergency line away from the truck. As soon as the
emergency line rips away from the truck the air in the line goes to zero
which causes the valve on the trailer the emergency line is attached to to
cut off charging air to the air tank on the trailer and supply max air
pressure from the air tank to the master brake cylinder.
Jim
At 10:40 PM 12/25/2003, Greg Anderson wrote:
>I've got a question.Since the emergency applies air all the time and the
>service applies only when the brakes are applied,how does this valve know
>when the trailer breaks away?Greg
>----- Original Message -----
>From: <ke6myk@sbcglobal.net>
>To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
>Sent: Wednesday, December 24, 2003 9:19 PM
>Subject: Re: [MV] trailer brakes
>
>
> >
> > Rich Sherman <richs@volcano.net> wrote:
> > > At 02:42 PM 12/24/2003, Trish & Carla wrote:
> > > >Doesn't loss of air to the air brakes cause the brakes to be
> > > >applied?
> > >
> > > Only if full air brakes and they are the 'spring brake' type. That's
>where
> > > the air holds against the springs in order to release the brakes. That's
> > > the air release you hear when the parking brake is applied. A lot of
>the
> > > mil trailers are air over hydraulic where the air pressure applies the
> > > brakes, no air then no brakes.
> >
> > I've seen two styles of air-over-hydraulic systems on M105-series
> > 1.5-ton 2-wheel trailers. One style (older, I presume) has one air
> > line which is connected to the service connection. When the truck
> > brakes are applied, air pressure on this line actuates the master
> > cylinder on the trailer. I don't think there's any break-away system.
> >
> > The other style (newer, I presume) has two air lines. The service line
> > acts as described above. The emergency line fills an air tank on the
> > trailer through a one-way valve. If pressure is lost on the service
> > line, a pressure-sensing valve uses the pressure stored in the
> > trailer's tank to actuate the trailer's master cylinder.
> >
> > Unlike true air brake systems with spring brakes, the latter style
> > still has manually-applied mechanical parking brake levers. The
> > break-away system isn't suitable for parking a disconnected trailer,
> > because eventually the pressure in the trailer's tank will bleed off,
> > and the hydraulic brakes will no longer be applied.
> >
> > I would really prefer to have true air brakes with spring brakes on my
> > green trucks and trailers, but that's not how most of my vehicles were
> > designed. Sigh.
> >
> > --
> > Mark J. Blair, NF6X <ke6myk@sbcglobal.net>
> > PGP 2.6.2 public key available from http://www.keyserver.net/
> > Web page: http://www.qsl.net/ke6myk/
> >
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> >
>
>
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James Shanks
n1vbn@bit-net.com
1998 IMZ 8.103
1984 H-D FLHT-C
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