SERVICE REPAIR
H4.00XM-6 H4.00XMS-6 Europe [L005]
1800 SRM 734
Description and Operation
General
This section has a description and the repair procedures for the parts of the hydraulic brake system. These
parts include the brake booster, master cylinder, and brake shoe assemblies.
Description and Operation
The brake pedal is connected to the push rod for the
brake booster. The brake booster helps increase the
force on the master cylinder by using vacuum from
the engine. The piston in the master cylinder forces
oil to the wheel cylinders to apply the brakes. (The
system allows braking without vacuum from the en-
gine.)
1. PUSH ROD
2. BRAKE BOOSTER
3. RESERVOIR
4. MASTER
CYLINDER
5. OUTLET
TO WHEEL
CYLINDERS
6. ENGINE VACUUM
CONNECTION
Figure 1. Brake Booster and Master Cylinder
BRAKE BOOSTER AND MASTER
CYLINDER
The brake booster and master cylinder are assem-
bled as a unit. The brake booster has a diaphragm
that is actuated by engine vacuum. See Figure 1.
The diaphragm, along with the operator’s effort,
pushes on the piston in the master cylinder. Hy-
draulic pressure actuates the wheel cylinders as
the piston pushes fluid through the check valve. A
return spring pushes the piston back to the stop
plate when the pedal is released. Fluid can flow
through passages in the piston and past the pri-
mary cup as the piston returns. Fluid flows across
the piston to prevent a vacuum while fluid returns
through the check valve. When the piston passes
the compensator port, excess fluid from the system
can return to the reservoir. The check valve keeps a
very small amount of pressure in the brake system.
The reservoir for brake fluid has a sending unit for
a low fluid level warning light.
SERVICE BRAKE ASSEMBLY
A service brake assembly is installed at each end of
the housing for the drive axle. Each service brake as-
sembly has a single-wheel cylinder at the top of each
back plate. The support plate has an anchor for each
shoe. When the wheel cylinder