Dodge ram truck 3500 1996 Service Manual page 189

B R
BRAKES i - S
DESCRIPTION AND OPERATION (Continued)
Booster vacuum level is monitored by a warning
switch. The switch consists of a vacuum chamber
that measures vacuum level and a sensor in circuit
with the red brake warning light.
The vacuum chamber is connected to the booster
check valve by a vacuum supply hose. A wire harness
connects the switch sensor to the brake warning
light. I f booster vacuum falls below 8.5 Inches for
8-10 seconds or more, the switch sensor completes'
the circuit to the warning light causing i t to illumi­
nate.
1ASTER CYLINDER
BR models are equipped with a two-section master
cylinder assembly. Major components are the alumi­
num cylinder body and a removable plastic reservoir
(Fig. 5).
The primary and secondary pistons are located in
the cylinder body. Grommets are used to secure and
seal the reservoir in the cylinder body. The reservoir
cover is equipped with diaphragm seal and is a snap
fit on the reservoir.
The cylinder body is equipped with a quick take-up
valve located in the primary piston inlet port (Fig. 5).
The valve provides an extra volume of low pressure
fluid during Initial brake apply. Extra fluid volume
provides faster take-up of normal clearances in the
calipers and wheel cylinders.
The cylinder body, piston assemblies, reservoir,
grommets, seal and cover are all serviceable parts.
The take-up valve is the only non-serviceable compo­
nent. The valve is available only as part of a com­
plete cylinder body assembly.
APPLICATION
Three master cylinders are used i n BR models. The
differences between' them concern cylinder bore size
and fluid reservoir capacity.
• The master cylinder in 1500 models has a 28.6
mm (1.125 in.) bore.
• The master cylinder in 2500 and 3500 models
has a 31.8 mm (1.252 in.) bore.
Although the master cylinders used in 2500 and
3500 models have the same bore size, they have dif- .
ferent piston stroke lengths and fluid reservoir
capacities.
NOTE: Master cylinders and reservoirs must not be
interchanged. This will cause unsatisfactory opera­
tion and improper brake balance.
IDENTIFICATION CODES
The master cylinder and reservoir are coded for
identification. Code letters are provided on the cylin­
der body and reservoir for proper component match­
ing.
PRIMARY
PISTON
ASSEMBLY J9405-13
Fig. 5 Master Cylinder
The cylinder body code letters are etched into the
front end of the cylinder. The reservoir code letters
are on an adhesive backed tag attached to the front
face of the reservoir (Fig. 6).
Code letters are as follows:
* 1/2 ton (Model 1500) code: NK
• * 3/4 ton (Model 2500) code: NL
• 1 ton (Model 3500) code: NP
COMBINATION VALVE
Pressure Differential Switch
The pressure differential switch is connected to the
brake warning lamp. The switch is triggered by
movement of the switch valve. The purpose of the
switch is to monitor fluid pressure i n the separate
front/rear brake hydraulic circuits.
A decrease or loss of fluid pressure in either
hydraulic circuit will cause the switch valve to shut-