From: Greg Anderson (glanderson@alltel.net)
Date: Thu Dec 25 2003 - 19:40:30 PST
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/
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>
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