HOW DO BRAKES WORK?
A typical automotive brake system is made of disk brakes or drum brakes. Disk brakes are located in the front of the vehicle; drum brakes in the rear and are connected to the master cylinder at each wheel via tubes and hoses. One-hundred years of modification and improvement of the brake systems we use in our cars today ensures that the brakes we have today are as dependable and efficient as history has allowed. Parking brakes, power brake boosters, and anti-lock systems are some of these modern modifications.
Your foot acts makes this network of tubes and hoses connect to the brakes. Stepping on the pedal pushes a rod into the master cylinder, which then pushes hydraulic oil (brake fluid) into these tubes and hoses. The brake fluid travels to each braking unit at each wheel. Hydraulic fluid can maintain its shape and pressure, while moving through the piping channels. Through its many twists and flips, it maintains its motion. /Some problems associated with the proper functioning of brake fluid and brakes: air bubbles that can take away some of the original energy of the brake fluid as it travels, as well as compressed air that makes it more difficult to operate the brake pedal. If air is present, the in system must be “bled” to remove the air, using the “bleeder screws” located at each wheel cylinder and caliper.
HOW ARE THE PARTS OF THE BRAKE SYSTEM ASSEMBLED?
On a disk brake, the fluid from the master cylinder moves into the caliper, pressing against a piston. The two brake pads are squeezed against the disk by the motion of the piston. The brake pads are forced to slow down or to stop completely, as they are squeezed against the rotor. The rotor is attached to the wheel, connecting the pressure of the brake pads to the motion of the wheel. And voila – forward motion can be stopped!
HOW IS THE DRUM BRAKE SYSTEM DIFFERENT FROM THE DISK BRAKES?
Rather than using a piston to stop the motion of the wheels, drum brakes use the pressure of brake fluid to cause enough friction between the drums and the wheels to stop or slow spinning. Brake fluid moves into the wheel cylinder, pushing out the brake shoes until the linings are pressed against the drum. The drum is hooked to the wheel. It is the friction of either the disc or the drum brakes that stops the movement of the vehicle. Friction, as many know, creates heat. This is why brake pads have to eventually be replaced – the forward motion of the car is transferred to the pads in the form of heat. Eventually, this heat takes its toll.
WHAT ARE OTHER PROBLEMS THAT MIGHT OCCUR WITH THESE SYSTEMS?
The master cylinder compartment is located directly in front of the driver’s seat in the engine compartment on the firewall. It typically contains two small master cylinders – one for each wheel. If one brake fails, the warning light on the dash will alert you. Master cylinders do not often experience problems, but an internal leak can occur, in which case the brake pedal may sink to the floor, when you attempt to push down with your foot. Releasing the pedal and immediately re-applying pressure by stepping down will bring the pedal back to its raised, resting position. The car can still be operated for a short time, as long as one of the brakes is in working order.
WHAT DO I NEED TO KNOW ABOUT BRAKE FLUID?
Brake Fluid is a special kind of oil that is located on top of the master cylinder. Brake fluid is a type of oil made to withstand the cold and the heat; it does not thicken or boil in these conditions. The Department of Transportation sets national standards for brake fluid to ensure that boiling or freezing does not occur. DOT-3, the current standard, has a boiling point of 460 degrees F. The owners’ manual for your vehicle may contain a specific recommendation from the manufacturer, however, which is worth consulting. The high boiling point of brake fluid is very important for the proper function of the brakes. Exposure to air will lead to some absorption of atmospheric moisture. This lowers the boiling point and increases the chance of brake failure. Using substances other than recommended brake fluids can also lead to brake failure. The rubber seals in the brake system can be destroyed by the mixing of other substances, so stick to the recommended oils! /Checking brake oil levels is easy to do in most cars. You often do not even need to lift the hood, as there is the fluid reservoir on top of the master cylinder is usually transparent. The fluid levels will drop over time with brake use. Although, if levels drop at a faster rate in a very short amount of time, it is good to have the brakes checked for problems.
WHAT ARE BRAKE LINES?
The lines from the brake fluid reservoir to the brake pads is made up of non-corrosive steel tubes and flexible, reinforced rubber hoses. The steel lines in the system may need to be completely repaired if damage occurs, depending on the damage. Replacing the entire line is typically the best option, but special splice fittings can also be used if a completely new line is too difficult. Brass compression fittings or copper tubing are not acceptable methods of repair of steel lines. They are illegal, because they are dangerous!
EQUALIZING VALVE OR PROPORTIONING VALVE
Proportioning or Equalizing Valves are mounted between the master cylinder and the rear valves. These valves balance pressure levels between the front and rear brakes, depending on how fast you were going and how fast you need to slow down. If you stop very quickly, the front of the vehicle will dive and the rear will lift slightly. Balancing the pressure between the front and rear when stopping prevents lockup in the rear wheels that may prevent the brakes from slowing the wheels effectively.
PRESSURE DIFFERENTIAL VALVE
The Pressure Differential Valve is mounted below the master cylinder in most cases and is in charge of detecting a malfunction and altering the driver by activating the brake warning light. It measures the pressure from the two sections of the master cylinder and checks for equilibrium. A difference in levels indicates a leakage of brake fluid. The combination valve acts as a proportioning valve and pressure differential valve together. Two functions in one!
ELECTRONIC BRAKE FORCE DISTRIBUTION
An Electronic Brake Force Distribution (EBD) replaces proportioning valve systems, distributing exactly equal amounts of pressure to each wheel. The result – a perfectly balanced brake system.
Disk brakes are some of the best brakes out there. They are capable of stopping big rigs and airplanes to minis and smart cars. They tend to stick less than other brake models and have a longer life-span, less affected by the elements and the muck of the air. Brake pads, a rotor, caliper, and a caliper support are the primary parts of a disk brake system.
There are two brake pads on each caliper and are made up of a metal “shoe” with a riveted or bonded lining within the shoe. Brake pads wear out over time and need to be replaced. They should be checked for replacement periodically. If the metal shoe is visible, meaning the pad is almost completely worn off, they must be replaced right away. Metal-to-metal contact is harmful to the rotor and to the wheel. A “brake warning sensor” – a sound that alarms when the pads are down to dangerous levels due to wear – is a good indicator that replacement is immediately necessary. The quality of the pads will depend with the type chosen. A more durable lining typically has a longer brake life, as well as a faster stop time. They may, however, squeal quite harshly when put to the test. So, noise and durability are equal concerns, sometimes difficult to rectify.
Another part which experiences a lot of wear and tear in a brake system is a rotor. The rotor comes into direct contact with the brake pads, so its patterns of wear will mirror those of the brake pads. The area of the rotor that touches the brake pads must be a prepared metal surface. A minimum thickness of measurement is ensured by every technician. Below this legal minimum of thickness, the rotor will not be able to function properly. The brakes will not be able to absorb the heat energy created from the friction of the pads against the wheels. Stopping can become quite difficult in this case!
Floating calipers and fixed calipers are the two main types of calipers. More specifically, Single Piston Floating Calipers are the most popular and lease expensive to produce and repair. It is called “floating” because it moves within a track to center itself over the rotor. This is opposed to the “fixed” forms of calipers. It is pushed by the hydraulic fluid in two directions by the push of the brake pedal, being forced against the inner pad and against the outer pad. The inner pad pushes against and thus activates the rotor. The outer pad pushes against the other side of the rotor. These floating calipers may also be located on the same side of the vehicle. They are called two piston floating calipers, and are found on more expensive cars. The sensation of more effective braking is achieved with the two piston form. Four Piston Fixed Calipers are not mobile like the floating calipers; there are two pistons on each side that press the pads against the rotor, without any movement by the caliper. Although more efficient, fixed calipers are more expensive all-around. Expect such a caliper on a Rolls Royce, rather than a Subaru.
WHAT ABOUT DRUM BRAKES?
Drum brakes are still around because they tend to be less expensive, when adding a parking brake. Parking brakes are entirely mechanical and cannot be operated hydraulically. Just a lever is needed with drum brakes for a parking brake to be installed, as opposed to a completely new mechanical system for a car with disc brakes. Drum brakes are made up of a backing plate, brake shoes, brake drum, wheel cylinder, return springs, and an automatic system. Brake fluid moves into the wheel cylinder upon foot pressure on the brake pedal. The wheel cylinder pushes the brake shoes into the machined surface on the inside of the drum. Removing the foot from the brake makes the return springs pull the brake shoes back to their original resting position. As the surface wears down with usage, the two parts of brake shoes and drum move slowly away from each other. The automatic system corrects this by constantly re-positioning the resting position of the brake shoes within equal distance to the drum. The brake shoes of the drum system are similar to the disk system – a lining is bonded or riveted to a steel shoe. These linings must also be replaced with time to prevent metal-to-metal contact and friction.
A piece with often little need for service or repair is the backing plate is the part which holds everything together, attached to the axle and forming a resting surface for the wheel cylinder, brake shoes, and assorted hardware. /Brake drums are like disk rotors in that have a machined side with which the brake shoes make contact. Maximum diameter (like maximum thickness with disk brakes) are stamped on the outside of the drum. They cannot exceed this level.
The wheel cylinder has one piston on each side of the vehicle, so two in total, each with a rubber seal and shaft that connects the piston to the brake shoe. As the foot pushes down on the brake pedal, the pistons push the shoes into contact with the drum.
The only problem associated with wheel cylinders is potential leakage. When the foot pulls back, return springs adjust the shoes back to their resting position. The springs must be in top function, in order to prevent fatigue of the lining. This can happen if the shoes are not pulled back completely by weaker springs.
A well-functioning self-adjusting system is important to keep the reaction time of the brakes to a minimum. This means, that the more wear incurred by the self-adjusting system, the less-responsive your brakes will be; the brake shoes will be further and further away from the drum. The self-adjusting parts can be cleaned and lubricated by a technician, when the brake system is serviced.
The parking brake or emergency brake controls the rear brakes through a series of steel cables connected to a hand lever or a foot pedal. It is a mechanical system, meaning it can kick-in to slow down your vehicle in the event of a hydraulic failure. With drum brakes, cable pulls on a rear brake lever to manually stop the vehicle, by directly slowing the brake shoes. With disk brakes, the design makes adding a parking brake difficult, as mentioned, but there are two modifications that can be made. Firstly, you can add a lever attached to a mechanical corkscrew device, which is housed in the caliper piston. This lever utilizes the already present rear wheel caliper to stop the vehicle and is used predominately with single piston floating calipers, so long as they are not fixed. Secondly, you can mount a mechanical drum brake unit to the inside of the rear rotor. The “drum” referred to here is the interior part of the rear brake rotor; its brake shoes are activated by the pulling of a lever. /Automatic vehicles do not often require the usage of a parking brake, which can actually create problems. This is because disuse of a parking brake leads to a build-up of corrosive materials. When going to use the parking brake, the cables may seize up. The self-adjusting mechanism on the parking brake may also not be keeping the shoes aligned due to a lack of use. Using the parking brake every now and again keeps the brake lines free of gunk and the alignment system working properly to ensure the efficiency of your brakes.
POWER BRAKE BOOSTER
The power brake booster can be found on the firewall behind the master cylinder. It is connected to the brake pedal so that it amplifies the available foot pressure applied. The booster is bi-chambered shell, separated by a rubber diaphragm. A valve in the diaphragm opens when your foot lifts off the brake pedal, allowing engine vacuum to enter the chambers. This engine vacuum, which powers the booster, is produced as a result of the engine running. Its power enters the booster through the booster’s check valve, which is then channeled to the engine via a rubber hose. This hose only allows the vacuum to enter but not to escape, trapping the power for the booster’s use only. Power boosters experience little problems, but a few considerations are worth noting.
Firstly, the engine must be running in order for power assist to work due to its reliance on vacuum generated by the engine. If the engine stalls or shuts off while in motion, you will have a few presses on the pedal left on the brake before it will become very difficult to stop. Pressing the pedal to the floor may be required.
ANTI-LOCK BRAKES (ABS)
If you are driving fast and need to suddenly brake, you may slap your foot down on the pedal. Oftentimes the wheels can lock up, the tires will screech, and the stop-time will not be as fast as desired. As many know when applying brakes in icy conditions, you can lose control of the vehicle’s steering system. Skidding on the road when braking abruptly can also burn off some of the tire’s rubber, resulting in a “flat spot” in the tread. Thumping along while driving is not anything anyone wants. These problems can all be prevented by an Anti-lock brake system.
So how does ABS work? Anti-lock brake systems prevent lock-up by pumping the brakes to prevent any sticking and skidding. You can keep full control of your control and stop quickly, relying on ABS to do the work for you. Pumping your brakes manually is actually much slower than ABS by ten or more times a second. This is a critical time difference, in terms of safety and prevention of an accident. The anti-lock brake system can do more than the human foot can to prevent locking of wheels.
The system consists of an electronic control unit, a hydraulic actuator, and wheel speed sensors on each wheel. It uses a computer in the control unit to monitor the speed of each wheel, pumping on the wheel that is locked, in most cases, in order to keep other wheels working well. The computer sends a signal to the brake system to remove and reapply/pulse the pressure to the locked wheel, when one or more wheels have stopped efficiently rotating. You can expect to feel a strong vibration from the brake pedal, when ABS begins to kick-in. If the control unit detects a problem in the ABS system, a warning light for the ABS will appear on the dashboard. Problems with ABS will not affect the normal function of your brakes, but should get checked out, nonetheless, to prevent locking-up of wheels. To find out if your vehicle has ABS, consult your owner’s manual.
Exposure to the outside air exposes your brake fluid to moisture. Brake fluid is hygroscopic – it pulls in and absorbs water molecules from the air. The rubber tubes and brake cylinder reservoir allow water to enter the system.
WHY IS WATER BAD FOR YOUR BRAKES?
The boiling point of brake fluid is lowered with a higher water content; a lowered boiling point means an increased risk of improper function of the brake system, when you need it most. This is because when brake fluid boils, it releases vapors. These vapors disrupt the pressurized system that allows for brakes to quickly and efficiently function. When you press down on the brake pedal after the fluid has passed its boiling point, it may be completely depressed to the floor! Not a very safe situation, indeed. For this reason, it is important to service your brake fluid regularly. There is another problem associated with moisture contamination of brake fluid that is cause for frequent brake service, particularly if you have ABS type brakes. With ABS systems, high temperatures of brake fluids, which are reached more rapidly with the lowered boiling point due to water content as just explained, raises the acidity of the fluid. The fluid then becomes corrosive, damaging the steel lines, rubber hoses, and cast iron or aluminum components of your brakes. Preventing this moisture in the first place by a brake fluid change will reduce the risk to the entire brake system. Let our trained technicians at 7 to 7 Auto Services perform this important safety service for you today.
Call us at (858) 549-0272 for an appointment.
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