Today’s Automotive Brake Systems
The braking system in today’s automobile like many other systems that I have written recently about have advanced tenfold. System basics have not changed much as we are still using hydraulically operated calipers and wheel cylinders to either squeeze brake pads against a spinning rotor or push brake shoes against the inside of a spinning drum until the vehicle slows down and comes to a stop Where the advancements have come is in the area of electronics and integration of the braking system into other systems such as ESC systems (Electronic Stability Control), Crash Avoidance Systems, Automatic Cruise Control and even Wind Stabilization Systems for some of the larger passenger and work vans hitting the market.
Most drivers are familiar with the ABS (Automatic Braking System) but, most have very little insight as to its operation other than the yellow or red ABS light coming on in the dash when the vehicle is started and hopefully goes off after a few seconds. This is a self-test feature of the system to let you know the system has passed, at least electrically. Some systems will cycle the ABS pump at a slow speed when you begin driving to further test the system. If the ABS light comes on and stays on, it means it’s time to have the system inspected.
The ABS is designed to shorten vehicle stopping distance and with the addition of ESC systems help to control vehicle stability in a panic stop, skid or swerve. This is done by measuring the speed of each wheel, yawl rate, steering angle and brake pressure to calculate a strategy in preventing wheel lock-up or wheel speed reduction at each wheel. Valves in the ABS module open and close as many as 100 of times per second or more to achieve the proper pressure at each wheel for a given situation. In addition to the pressure provided by the brake master cylinder through the driver’s foot pressure on the brake pedal, a high pressure pump is also used to add pressure to the cycling valves if determined it’s needed during ABS operation. The ABS is networked with the other systems using its capabilities at a very high baud rate to meet its operating needs. When the system is called upon to operate, the driver will feel a slight pulsation in the brake pedal and noise from the ABS pump operation. It is important that the foot pressure on the brake pedal stay constant for the system to operate correctly. I often recommend trying to operate the system on sand, loose gravel or wet or icy pavement to feel how the system will react and feel how the pedal may react so they will know better what to expect in a panic situation.
So, what does all that mean to the vehicle owner? In addition to monitoring the ABS light operation, regular brake inspections and fluid flushing is extremely important. Brake fluid is hygroscopic, meaning it absorbs moisture. Over time brake fluid will deteriorate and absorb moisture. This will lead to corrosion in the hydraulic system and if high enough will lower the boiling point of the fluid. This may lead to brake fade, a serious condition. Testing for moisture content has improved with better systems for detection. Tools determining the actual boiling point are the most accurate. Proper fluid flushing is essential to keeping this system clean and it should be done before the fluid gets dirty. Like coolant, brake fluid has corrosion inhibitors added to extend the life of the fluid and more importantly, protect the brake system. When brake fluid has begun to deteriorate, the first components affected are the brake lines. The brake lines are mostly made from copper. Test strips are dipped into the brake reservoir to measure the copper content of the fluid. If above 200ppm the fluid should be flushed. When brake fluid begins to or has turned black, the rubber components such as brake hoses have begun to deteriorate internally and need to be replaced and the system flushed as well.
Brake fluid level in the master cylinder is designed to be an indicator of brake pad and shoe wear. If it is topped off during a maintenance check or service it will not be able to be used as a true indicator of brake lining wear. As pads and shoes wear, the caliper and wheel cylinder pistons extend, reducing the fluid level in the master cylinder. Most master cylinder reservoirs have a level sensor that will let the driver know by turning on the red brake light on the dash when the fluid is too low. Don’t just add fluid and continue driving. Some cars have brake pad wear indicators that will cause a squealing noise at the wheel when moving, some will have an electronic sensor and some will have none at all. The last may cause a metal to metal condition of the brake pad backing rubbing on the brake rotor instead of pad material. A very unsafe condition, for several reasons.
When brake pads or shoes are in need of replacement, it is so important to use a premium product. It is said, “you get what you pay for” and enough can’t be said about brake pad choice. It is important is the pad or shoe material meets or exceeds manufacturer specs so the ABS and other systems counting on that are able to perform correctly for safety. As I said earlier, all these systems have to work together. Using a low cost pad that may fit the vehicle but, how do they perform when you really need them? Caliper seals, slides and hardware should always be inspected, cleaned, lubricated or replaced during pad replacement or notice of uneven brake pad wear. Sometimes calipers may need to be replaced because they are leaking hydraulically from a worn seal or have sticking pistons that apply the brake pads. Master cylinders also can leak and will mostly leak at the point where the piston is pushed. Most times the leak won’t be visible as it will leak brake fluid into the vacuum booster attached to the back of the master cylinder causing an early booster failure due to vacuum diaphragm contamination. This will cause a needed increase in brake pedal foot pressure to apply the brakes and make it feel harder to stop the vehicle.
Other items that can affect the braking systems and related systems performance are shocks, struts or springs. Shocks, struts and springs are important because if they are not operating correctly, the systems that depend on them can’t function properly as well. If a vehicle’s nose dips excessively from worn shocks, struts or springs it will put extra pressure on the front brakes, shortening their life and extend the stopping distance as much as 12 feet in panic stop. If a vehicle sways too much in cornering and the ABS is needed, the ESC system may not be able to determine the best course of action to apply and re-stabilize the vehicle.
I realize that I covered a lot of material in this addition. Brake system operation, service and safety should never be taken lightly. We have trained technicians with more than 25 years of experience each and the needed equipment to keep of your brake and all related systems working properly. As always, you can contact us with any questions regarding brake related service or repair.
Kevin Hough, Pres
ASE Certified Master Technician and L1 Advanced Level Specialist