GMC 4000 Series Service Manual page 239

Sec. 5B
Page 232
AIR BRAKES
On tractors which have an air operated em er­
gency brake system (such as DD3), as opposed to
those having a mechanically applied emergency
brake, this valve is used to apply both tractor and
trailer emergency system s simultaneously. The
basic purpose of this type application is to prevent
trailer "jack-knife" conditions, which can result
from applying only tractor brakes while vehicle is
in motion.
The Synchro valve is mounted in the air line
system controlling the trailer emergency valve. It
is installed in the lines at the cab front cross sill,
in or near the junction block in the sill. It is ac­
cessib le with cab tilted, or from underneath with
cab in normal position. The supply of air from the
Synchro valve is connected to the control port of
the tractor protection valve.
SERVICEABILITY TESTS
1. Operating Test. Pull knob of push-pull con­
trol valve (tractor emergency brake) and note that
trailer emergency brakes also apply. Push knob of
push-pull control valve and note that trailer em er­
gency brakes release. A 100 psi foot valve brake
"Anchorlok" brake chambers are used as
optional equipment on all 5500 and 6500 Series
trucks. Refer to figure 22.
The "Anchorlok" chamber is used as a service
brake chamber, an emergency brake in case of air
pressure lo ss and a spring-applied parking brake.
It consists of two separate air chambers, each
having its own diaphragm and push rod. In the front
chamber, air pressure enters behind the diaphragm
when brake pedal is pushed, causing a service
brake application, just as in any standard chamber.
The rear chamber is subject to constant air p res­
sure in front of the diaphragm, compressing the
emergency parking spring. In the event of lo ss of
Figure 2 2 — A ncho rlok Brake C h am b er Installed
G M C S E R V I C E M A N U A L
application will be necessary to complete release
of tractor brakes.
leakage both with brakes applied and released. If
any leakage is evident, remove valve from vehicle
and repair or replace.
REPLACEMENT
Removal
brake application (with DD3 system , parking brake
is effective after air is exhausted).
and remove valve.
Installation
least 100 psi.
a service brake application. This will release the
parking brake application.
ANCHORLOK BRAKE CHAMBERS
air pressure in the rear chamber, or intentional
exhausting of pressure by the driver, the spring
will apply the brakes. Application will begin when
pressure drops to approximately 45 psi and will
be complete when pressure reaches approximately
25 psi.
2. Leakage Test. Using soap suds, check for
1. Block vehicle wheels or make a parking
2. Exhaust air from brake system .
3. Disconnect air lines from synchro valve
1. Place valve in position and connect air lines.
2. Start engine and build up air pressure to at
3. Push "in" on control valve knob and make
4. Test as directed in applicable procedure.
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STUD, NUT, A N D
W ASH ER INSTALLED
(SPRING COMPRESSED)
Figure 2 3 — Spring C om p re ssing A sse m b ly
STUD. NUT, A N D
W A SH ER STO W ED
AT SIDE OF CHAMBER