Thursday February 6 2014

Why do my brakes release a lot of soot?

Thank you for the weekly knowledge and advice you give to drivers. I drive a Toyota Gaia. I keep noticing black soot coming out from the inside of the front wheels, indicating something wrong with the brake pads or some other parts in that area. What could be the real problem and how can it be solved?
Twine S D.

The black soot or brake dust you see on your Toyota front wheels is friction material from some types of brake pads and the quality of steel disc rotors. During the process of braking your Toyota, you turn forward energy into heat energy. This causes energy and heat dissipation in the brake system which results in brake pad or disc surface tension and resultant wear. One way to look at it is brakes are similar to a pair of new shoes which will never wear out if you keep them on your shoe rack. However, different brake systems generate varying amounts of soot depending on two main factors the quality of brake pads used or condition of discs on your wheels.

There are mainly two types of brake pad material that cause soot on the Ugandan market: the organic type (pressed pad material that disintegrates easily) and semi metallic which lasts longer and produces less soot. Ceramic brake pads are a special type which produce the least dust or soot but tend to have a little longer braking distance. Black dust on front wheels can also be disintegrated metal from warped or grooved brake disc rotors. Brake discs are sometimes warped or they develop grooves if poor quality semi-metallic brake pads are used and pads are not replaced on time.

On your Toyota, you can mitigate this problem by avoiding the very cheap brake pads and trying to buy genuine ones from leading independent Toyota parts dealers. Ask about the material composition of brake pads you intend to buy. To reduce brake dust from discs, skim the discs within acceptable width with a lathe machine to eliminate grooves. You can use sand paper to reduce soot during brake service or out rightly replace them if they wear beyond recommended width.

My car (ML 320) brakes seem bad, whenever I am on a slope, it takes long to brake, even when I press the brakes completely down. Recently, we replaced the brakes but in a week’s time it re-occurred.

We did service but there is no great difference. In the event that the brakes fail and it is a slope what should one do?
Jerusha Ariko

From your description it seems your Mercedes Benz ML brake booster may have failed. The main symptom of brake booster failure is a high, hard pedal that takes greater than normal pedal pressure to stop the car. It takes a much longer distance than normal to stop your car. The high, hard pedal and longer stopping distance mean you are not getting power boost. This component of your brake system uses engine vacuum to provide brake assist to the master cylinder.

The brake booster is designed to create a greater braking force from a minimum pedal effort using a difference in atmospheric pressure and the engine’s manifold vacuum. Pedal force is increased two to four times depending on the diaphragm size. The booster is located between the brake pedal and master cylinder. There is one simple test you and your mechanic can carry out to diagnose whether your brake booster has failed.

The booster operating check: with the engine stopped depress the brake pedal normally several times. The brake pedal must be depressed before the engine is started in order to remove vacuum from the booster. With the brake pedal depressed start the engine. When the engine is started, vacuum is created and operates the booster. This causes the brake pedal to go down. If the brake pedal goes down slightly the booster is operating normally. If the brake pedal does not move the booster is not receiving manifold vacuum or is malfunctioning.

If the booster passes this test let me know and I will give you steps for the air tightness test. Brake failure will often not happen suddenly. There are always clues that brakes are failing long before it occurs. It is important stop and have the brakes checked to avoid total failure rather than practice how to stop the car when they fail. I cannot predict the conditions around you if your brakes failed nor predict how you would react.

In the event that my brakes failed I would try to remain calm, keep my eyes on the road and look out for traffic or pedestrians while I hoot to warn them. I would remove my foot from the accelerator and pump the brakes several times to try and rebuild brake pressure. Then I would shift into a lower gear, easier done with a manual transmission. With the automatic transmission I would shift the gearstick to D1 to use as much braking assistance from low engine revolutions (sometimes called engine braking).