Brake Master Cylinder: Testing and Inspection

Usually, the first and strongest indicator of anything wrong in the brake system is a feeling through the brake pedal. In diagnosing the condition of the brake master cylinder, check pedal feel as evidence of a brake concern. Check for brake warning lamp illumination and the brake fluid level in the brake master cylinder reservoir.

NORMAL CONDITIONS
The following conditions are considered normal and are not indications that the brake master cylinder is in need of repair.
New brake systems are not designed to produce as hard a pedal effort as in the past. Complaints of light pedal efforts should be compared to pedal efforts of another vehicle, same model and year.
During normal operation of the brake master cylinder, the fluid level in the reservoir will rise during brake application and fall during release. The net fluid level (after brake application and release) will remain unchanged.
A trace of brake fluid exists on the brake booster shell below the master cylinder mounting flange. This results from the normal lubricating action of the master cylinder bore end seal.
Fluid level will fall with brake pad or shoe wear.

ABNORMAL CONDITIONS

NOTE: Prior to carrying out any diagnosis, make sure the brake system warning indicator is functional.

Changes in brake pedal feel or travel are indicators that something could be wrong in the brake system. The diagnostic procedure and techniques use brake pedal feel, warning indicator illumination and low brake fluid level as indicators in diagnosing brake system concerns. The following conditions are considered abnormal and indicate that the brake master cylinder is in need of repair.
Brake pedal goes down fast. This could be caused by an external or internal leak.
Brake pedal goes down slowly. This could be caused by an external or internal leak.
Brake pedal is low or feels spongy. This condition may be caused by no fluid in the brake master cylinder reservoir, reservoir cap vent holes clogged or air in the hydraulic system.
Brake pedal effort is excessive. This may be caused by a bind or obstruction in pedal/linkage, clogged fluid control valve or insufficient booster vacuum.
Rear brakes lock up during light pedal force. This may be caused by grease or fluid on brake pads or shoes, damaged brake pads or shoes, incorrectly adjusted parking brake.
Brake pedal effort is erratic. This condition could be caused by the brake booster, extreme caliper piston knock back or incorrectly installed brake pads or shoes.
Brake warning indicator is ON. This may be caused by low fluid level, ignition wire routing too close to fluid level indicator assembly, or float assembly damage.

BYPASS CONDITION TEST
1. Check the fluid in the brake master cylinder. Fill the brake master cylinder reservoir if low or empty.
2. Observe fluid level in brake master cylinder reservoir. If after several brake applications, the fluid level remains the same, measure wheel turning torque required to rotate wheels with the brakes applied as follows:
3. Place the transaxle in PARK or NEUTRAL and raise and support the vehicle on a hoist.
4. Apply the brakes with a minimum of 445 N (100 lb) and hold for approximately 15 seconds. With the brakes still applied, exert torque on the front wheels to 10 Nm (74 ft. lbs.). If either wheel rotates, install a new brake master cylinder.

NON PRESSURE LEAKS
Any empty brake master cylinder reservoir condition may be caused by two types of non pressure external leaks.
Type 1: An external leak may occur at the master cylinder reservoir cap because of incorrect positioning of the gasket and cap. Reposition cap and gasket.
Type 2: An external leak may occur at the reservoir mounting seals. Repair such a leak by installing new seals.