From: ke6myk@sbcglobal.net
Date: Wed Dec 24 2003 - 18:19:31 PST
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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