Auto Repair Help

by Lance Wright


The master cylinder is low on brake fluid or the brake fluid continually goes low.

USUAL CAUSEabs_light
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.

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.

(Lance owned his own auto repair shop for 30 years before retiring in 2006.)

3 responses to “Master Cylinder Is Low On Brake Fluid”

  1. Paul T says:

    I have an 89 toyota corolla all trac (146k ORIGIONAL miles from a minister!!! with dealership records) I scored the car from a trade in and friend of a friend for $450.00 and the body is perfect. The car has been sitting for the last two years because of….. A) me being in school. and B) what I was told, it had a bad master cylinder.
    I replaced the cylinder when I got the car. No problems. I was told by the friend that if the master ran low on fluid that it would have to be replaced……fine, the car is old so I did that. The car pissed the brake fluid out after 6 months and I replaced a wheel cylinder and brakes. I bled the car and it worked but the pedal did not seem right. My friend tried to bleed it as well and then told me I would have to replace the master cylinder again. He told me that if it ran low it was probably bad. He is ASE certified and I am a mechanical engineer student with 4 years of military automotive repair experience. I have the time to get this vehicle on the road now. I do not believe the master cylinder would be bad after 6 months. I am about to buy one way bleeder valves and try bleeding it again. Who is right here and what would you suggest for a student on a low income?

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