GMC 4000 Series Service Manual page 251

Sec. 5B
Page 244________________________
AIR BRAKES
GENERAL
The Stopmaster "Fail-Safe" is a mechanical
actuation feature for power brakes and is two-fold
in purpose. It is used as an air-released, spring-
applied parking brake, as well as a safety feature
in the event of air brake failure.
DESCRIPTION A N D OPERATION
BRAKE CHAMBERS (Fig. 40)
Two brake chamber assem blies are used at
each wheel. Type used differs by model as shown
in figure 27. The "Fail-Safe" brake chamber a s­
sembly consists of an inner and outer chamber.
The inner chamber, containing a diaphragm and
diaphragm plate rod, serves as the service brake
chamber and operates in the same manner as a
conventional brake chamber. Movement of dia­
phragm plate rod is transmitted to the brake shoes
In let Port
(Service B rakes)
1 Spring Lock
2 Release Bolt
3 Spring and Cap
Assembly
4 Release Bolt Nut
5 Piston
Pressure Housing
6
7 Clamping Ring
Rubber Diaphragm
8
9 Diaphragm Plate
Rod
10 Rubber Boot
11 Wedge Rod Guide
Figure 4 0 — " Fa il-Safe" Brake C ham ber A sse m b ly
G M C S E R V I C E M A N U A L
"FAIL-SAFE" BRAKES
4
Inlet Port
(P a rk in g Brake)
Wedge Rod (Part of
1 2
Wedge Assembly]
13 Non-Pressure
Housing
14 O-Ring
15 Rubber Washer
16 Spring
17 Rivet
18 Retainer Plate
19 Piston Seal
Expander
2 0
Spring Lock
2 1
Capscrew
T - 2 1 9 9
through the mechanical actuating components as
described later.
The outer chamber contains a spring-loaded
piston which acts against the service brake cham­
ber diaphragm plate rod. During normal operation,
constant air pressure is applied to the outer cham­
ber, this pressure, acting on the piston, holds the
spring compressed. When air pressure is released
from the outer chamber by pulling out the knob on
the parking brake control valve, spring pressure
forces the piston toward the service brake cham­
ber; piston movement is transmitted through the
service brake diaphragm plate rod to the brake
actuating mechanism, applying the brakes.
This same action will take place in the event
air pressure is lost from the system .
NOTE: The air tank supplying air pressure
to the parking brake chambers is protected from
the main air system by a one-way check valve;
this tank will contain enough pressure for at least
one parking brake release in case pressure is lost
from the main air system .
In case of complete pressure loss, and p res­
sure in the protected tank is depleted, brakes will
remain applied until air pressure is restored. If
necessary to move the vehicle before air pressure
can be restored, brakes can be released by turn­
ing the release bolts clockwise as far as possible
(approximately 18 turns) to com press the springs,
as shown in figure 42. (This must be done at all
"Fail-Safe" chambers at each wheel.) After air
pressure has been restored, service brakes will
be operative immediately; however, the parking
brake will remain inoperative until the spring re­
lease bolts are backed out (counterclockwise) as
far as possible to release the springs. Push park­
ing brake valve knob in to released position to ad­
mit air pressure into the parking brake chambers
to hold springs compressed while turning the re­
lease bolts.
CAUTION: Under no circum stances should any
service operations be attempted on the brake cham­
bers without first com pressing the springs by
means of the release bolts. Applying air pressure
(at least 60 psi) to the parking brake chambers,
either from the vehicle air system or from shop
air supply, will hold springs compressed and
facilitate turning bolts.
MECHANICAL ACTUATING COMPONENTS
(Refer to Fig. 33)
When pressure is applied to the brake cham­
ber diaphragm plate rod, either by air pressure
during a service brake application or by spring
pressure from the parking brake chamber, move­