Brake Fluid: Testing and Inspection

NOTE: It is recommended that GM PAN 12377967 brake fluid or equivalent meeting DOT-3 specifications be used in brake systems. Use of DOT-5 silicone brake fluid is not recommended, as excessive wear of internal brake system components may result. DOT-5 brake fluid should never be used in the Antilock Brake System. Brake fluid should always be stored in closed containers, as it will absorb moisture from the atmosphere. Improper brake fluid, mineral oil or water in the fluid may cause the brake fluid to boil or the rubber components in the hydraulic system to deteriorate. When checking the brake fluid level, the master cylinder reservoir may appear to be low if the front or rear linings are worn. This is normal.

SUBSTANDARD OR CONTAMINATED BRAKE FLUID
Improper brake fluid, oil or water in the fluid may cause the brake fluid to boil or the rubber components in the hydraulic system to deteriorate. If master cylinder piston cups are swollen, then rubber parts may have been in contact with improper or contaminated fluid. This deterioration may also be evidenced by a swollen master cylinder cap cover diaphragm or caliper piston seals. If deterioration of rubber is evident, disassemble all hydraulic parts and wash with alcohol. Dry these parts with dry compressed air before assembly to keep alcohol out of the system. Replace all rubber parts in the system including hoses. When working on calipers check for fluid on the linings. If excessive fluid is noted, replace the linings. The system must be flushed if there is any doubt as to the grade of fluid in the system or if fluid has been used which has been in contract with parts that have been subjected to contaminated fluid. The Antilock Brake System hydraulic modulator cannot be disassembled for cleaning. If the brake fluid in a vehicle becomes contaminated, the ABS hydraulic modulator may have to be replaced.

BRAKE FLUID LEAK CHECK
With engine stopped and the transaxle in neutral, pump the pedal several times until all vacuum boost is depleted. Depress the brake pedal and hold a constant foot pressure on the pedal. If pedal gradually falls away with the constant pressure, the hydraulic system may be leaking. Check the master cylinder fluid level. While a slight drop in reservoir level results from normal lining wear, an abnormally low level in the reservoir may indicate a leak in the system. The hydraulic system may be leaking internally as well as externally. Refer to MASTER CYLINDER INSPECTION Steps 1 and 2. The system may appear to pass this test but still have slight leakage.