DIAGNOSE – MASTER CYLINDER IS LOW ON BRAKE FLUID
by Lance Wright
The master cylinder is low on brake fluid or the brake fluid continually goes low.
It is normal for the master fluid level to decrease slightly as the brake pads wear. As the brake pads wear, the pistons in the caliper extend to compensate for the friction wear. As this occurs, the caliper will store more brake fluid in the caliper cylinder and will decrease the fluid level in the master cylinder.
You will need to determine whether the fluid level is low in the master cylinder due to a fluid leak or the normal brake pad wear discussed above. A brake inspeciton should be performed to determine the cause. The calipers, wheel cylinders (on drum brakes) will need to be inspected. The brake pads and shoes (if equipped) should be inspected for signs of brake fluid contamination indicating a leak. Inspect the calipers for leaks at the rubber dust boot around the pistons. The dust boot on the wheel cylinder will have to be pulled off slightly to inspect for leaks. Check the hydraulic lines, proportioning valve and flexible hoses for leaks. Inspect the master cylinder where it mounts on the brake boosters for signs of leaks. Loosening the master cylinder retaining bolts and pulling it back slightly, will usually result in detection of a leak if present. If leaking, the fluid will run out of the mating surfaces. In this case, the master cylinder will require replacement. If there are no leaks present and the pads are worn, the most likely cause of the low brake fluid level is worn brake pads.
PRECAUTIONS, TIPS, and NOTES
The brake fluid level should be inspected at every oil change. If servicing the hydraulic system, use caution not to contaminate the system with dirt, debris or water. After the hydraulic system is serviced it will require bleeding. Refer to the manufacturers bleeding procedure to ensure all of the air is purged from the system. Always use the recommended brake fluid DOT rating. This information can be obtained from your owners manual or off the cap on the master cylinder.
owned his own auto repair shop for 30 years before retiring in 2006.)