vehicle. If a stoplight switch is mounted on the
from around the ends of the cylinder to check for
cylinder, the wires must be disconnected first. Next,
leakage between the pistons and the cylinder wall.
the hydraulic line and the pedal linkage are
An operational inspection of a hydraulic brake
disconnected from the cylinder. Then, loosen the
system may be accomplished during the road test.
hold-down bolts and remove the cylinder. The exterior
However, several checks may be made by operating the
surface of the cylinder should be thoroughly cleaned
brake pedal while the vehicle is parked.
before disassembly. Figure 2-37 identifies the various
If the brake pedal bottoms when pushed down, a
components and parts of the master cylinder.
further check must be made of the system. This
After the exterior surface of the cylinder is
condition may be caused by worn brake linings, but
cleaned, remove the filler cap of the reservoir and pour
m o r e o f t e n b y a d e f e c t ive h y d r a u l i c s y s t e m .
out and discard the fluid. Be sure to follow the proper
Insufficient fluid in the reservoir is a prime cause of
procedure for disposal of the fluid. Clamp the master
this problem. If the fluid is low, the reservoir should be
cylinder, boot upward, in a vise, and then remove the
checked for external leaks.
boot and pedal rod. Next, remove the spring retainer
Internal leakage from the lines through the master
(snap ring). With this removed, the piston stop (a thick
cylinder may also cause the pedal to bottom. Air in the
steel washer) is free to be removed with the piston.
hydraulic system should be suspected when the
Remove the master cylinder from the vise and up-end
operation of the pedal is soft or spongy. The air must be
the cylinder to allow the piston, spring, and valve
removed to obtain a solid pedal. This process is called
assembly to slide out. The secondary cup, primary cup,
piston spring, and valve assembly should all be
replaced with new parts. These items are usually
The maintenance of hydraulic brake systems
available in a repair kit.
consists of correcting the problems found during
scheduled inspections and those that occur during
After the parts are removed, the cylinder walls
normal operations. This includes servicing the system,
should be cleaned and inspected. If there are deep pits
repairing and replacing tubing and flexible hose,
or scratches in the bore, the master cylinder should be
repairing the master cylinder and wheel cylinders, and
replaced. The cylinder bore may be honed with a
suitable stone to remove rust, scores, and shallow pits
and scratches. After honing a cylinder, be sure that all
Servicing.--As applied to hydraulic brake
abrasive dust is removed; then, lubricate the bore with
systems, servicing consists of checking the fluid level
new, clean brake fluid.
and adding necessary fluid to the master cylinder
reservoir. When adding hydraulic fluid, be sure it is the
Assemble the parts into the cylinder in the logical
kind recommended by the manufacturer. Some
order. First, install the valve assembly on the end of the
manufacturers use natural rubber seals in the operating
piston return spring. Check again to ensure that the
systems, and others use synthetic rubber or other
bore of the cylinder is clean and lubricated with clean
materials. Unless the recommended brake fluid is
fluid. Lubricate the new primary and secondary cups
used, the seals deteriorate quickly, causing possible
and piston. Guide the lips of the primary cup into the
failure of the brake system.
bore and use the piston to force the cup into the
Dirt and other foreign matter that accumulates
around the filler opening can also affect brake
As the secondary cup is installed, be sure that the
operation. Even a small particle of dirt may find its way
lips do not fold over when they contact the bore. Then,
into the operating mechanism and close a vent or
place the piston stop on the piston, and with a drift
prevent a valve from sealing properly. All dirt and
punch inserted in the hole at the back of the piston,
foreign matter must be removed before removing the
force the piston into place. Hold the piston in place and
filler cap. The fluid level should be approximately 1/4
install the spring retainer. Replace the pedal rod and a
to 1/2 inch from the filler opening. This distance is
new boot. Install the filler cap only after ensuring that
usually specified in the applicable service technical
the vent hole is open.
manual. This space compensates for thermal
Reinstall the master cylinder on the vehicle,
expansion of the brake fluid.
reconnecting the brake line and the wire to the
Master Cylinder Repair.--When the master
stoplight switch. Then connect the pedal rod to the
cylinder requires repair, it must be removed from the
brake pedal. The applicable technical manual should