Great Dane 42101401 - Manual (Page 12)

PARKING BRAKES
All axles (except some converter dolly axles) are
equipped with air/spring actuators. Each actuator is
separated into two units. The base unit applies the
service brakes. The top unit contains a coil spring that
must be compressed by air within the chamber to
release the parking brakes. Loss of air pressure in the
supply line to the brake chamber will automatically
apply parking and/or emergency braking.
In case of a service brake system air failure, when the
spring brakes are applied in an emergency stop, a
spring brake air reservoir retains enough stored air to
release the spring brakes at least once by means of
the tractor parking brake control.
In the absence of air pressure, a manual release is
provided to allow release of the spring brake (see
following instructions).
To manually release parking brake actuators:
1. Always position wheel chocks at both front and rear
of tires before manually releasing parking brakes.
2. A parking brake release tool may be stored in
a pocket on the side of the brake chamber.
(See photo.)
3. Insert the detachable release bolt through hole in
head. Turn the release bolt clockwise until it stops
and locks, then pull the release bolt out as far as
possible, and run the nut down, holding the bolt in
place.
Using a hand wrench, turn the release bolt nut
clockwise until the bolt extends about three inches.
Make sure the release bolt is locked properly in the
piston.
The parking brake coil spring is now caged.
BRAKE BALANCE
Differences in crack pressures on hold-off valves
on tractors and trailers can lead to air pressure
imbalances that cause the trailer brakes to do a
majority of the braking. Another contributing factor to
premature trailer brake wear is torque imbalances
created by differences in brake linings.
These imbalances can lead to greatly reduced trailer
brake lining life, trailer brake drum heat checking, heat
cracking and breakage.
The air pressure imbalance typically ranges from 4 to 6
psi between the tractor and trailer. The trailer industry
has been using 4 to 5 psi crack-pressure air valve
systems and tractor manufacturers in some cases are
using 8 to 10 psi hold-off valves.
This imbalance will cause the trailer to do nearly all the
braking in low-air pressure applications. And because
most normal braking is done at air pressure applications
of 10 to 15 psi, this means that routinely, the trailer
brakes are the ones that are stopping the vehicle in
all but the hardest braking.
One simple way to see if an imbalance situation exists
is to use a duplex gauge that measures air pressures
at the coupler and the actuator air chamber. This will
show if there is an imbalance situation between the
tractor and the trailer.
Another method is to use a special balance kit such as
the ones available from the major brake manufacturers.
The balance kits include gauges to check air pressures
in the trailer chambers, the tractor chambers and at the
coupler, and electronically compares them. Industry
standards recommend the tractor and trailer should be
within 1 and 2 psi.
DANGER DO NOT operate your trailer with
parking brakes caged or in any other way
disabled. Never attempt to open a brake
actuator. The internal spring is very dangerous.
Do not repair spring brakes. Discard only by
using proper, approved procedures. If this
procedure is not known, contact the actuator
manufacturer or Great Dane Trailers before
removing the actuator from the trailer. Never
operate the trailer with the end-cover cap removed.
WARNING FMVSS-121 revisions made
mandatory 10-8-92 require air system supply
lines to be protected to at least 70 psi. The 70
psi (multifunction) emergency air valves require
that tractor air compressor governors be set at
105 psi (minimum) to assure full release of the
trailer parking brakes.
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