GMC 4000 Series Service Manual page 217

Sec. 5B
Page 210
AIR BRAKES
1 Application Valve
2 Valve Mounting Bracket
3 Pedal Support Bracket
4 Pedal Return Spring
5 Pedal Stop Bumper
Figure 2 — Brake Pedal and Linkage (Cow l Models)
push rod is used between the brake pedal and the
brake application valve. On "L" models, an ad­
justable stop screw is provided to control the pedal
push rod to application valve relationship. These
adjustments are not normally required except when
the application valve, pedal, or push rod have been
removed and reinstalled.
On E6500 Models the brake pedal is part of the
application valve assembly and adjustment is by
means of a stop screw (Midland-Ross valve) or
a stop button (Bendix-Westinghouse valve).
PUSH ROD ADJUSTMENT
(5500, 6500 COWL MODELS) (Fig. 2)
Push rod adjustment controls the exhaust
opening between the hollow end of the application
valve piston and the exhaust valve, and the pedal
travel before brake application begins. If push
rod is too long, it will prevent the piston from
leaving the exhaust valve and brakes will not r e ­
lease, or if it does not open far enough it can re­
sult in slow brake release. If push rod is too short,
excessive pedal travel will be required to start
brake application, and full brake application may
not be obtained. With push rod installed (fig. 2),
adjust as follows:
1. Loosen lock nut on push rod.
2. Using a pair of pliers, grasp and turn rod
into rod end until pedal does not contact the pedal
stop bumper with push rod bottomed in piston cup.
3. Place a .040" shim between the brake pedal
and the pedal stop bumper, then adjust the push
rod until it just contacts the piston cup.
G M C S E R V I C E M A N U A L
6 Brake Pedal
7 Push Rod End
8 Adjusting Nut
9 Push Rod
10 Boot
T - 3 4 6 8
nut.
and release.
PEDAL STOP SCREW ADJUSTMENT
(TILT CAB MODELS - Fig. 3)
haust opening between the hollow end of the appli­
cation valve piston and the exhaust valve. It also
controls the pedal travel before brake application
begins. If stop screw is turned down too far, it will
prevent the piston from leaving the exhaust valve
and brakes will not release. If it does not open far
enough it can result in slow brake release. If stop
screw is not turned down far enough, excessive
pedal travel will be required to start the brake ap­
plication, and full application may not be obtained.
Adjust stop screw as follows:
between the push rod and the application valve
piston cup.
Push Rod to Pedal Pin
Push Rod
Application Valve
Valve and Pedal Support
Bracket
Application Valve Piston
Cup
Pedal Stop Screw
Figure 3 — Brake Pedal and Linkage (Tilt Cab M odels)
4. Hold push rod while firmly tightening lock
5. Check brake operation for full application
Pedal stop screw adjustment controls the ex­
1. Loosen lock nut on pedal stop screw.
2. Back stop screw out until there is free play
Lock Nut
Pedal to Bracket
Pin
Pedal 7,563