September 23, 2009
By Jim Kerr
Recently my mailbox has been filled with inquiries about braking problems. Many of these questions are related, and deal with brake noises, low brake pedal or pedal pulsation. This week, let’s take a look at one of the most important safety features on any automobile – the braking system.
The driver’s interaction with the brakes begins when you press the pedal. The pedal linkage pushes on a piston in the master cylinder, which pushes brake fluid through the lines and hoses to the wheels. A cylinder at each wheel has pistons (called a caliper on disc brakes) that changes the force of the brake fluid back into mechanical movement, pushing the brake pads against the rotating brake rotor and slowing the vehicle.
Hydraulic brake systems have proven reliable for decades, but there are still things that can go wrong. One of the most serious is a loss of brake fluid. The fluid moves through steel lines along the car body and through rubber flexible hoses at each wheel. Exposed to all the elements and flexed continually as the suspension moves up and down, the rubber hoses can crack or break. Fortunately, a visual inspection of the hoses will usually detect a problem long before it gets serious. Any hose with a crack on the outer layer of rubber or abrasion marks on it should be replaced.
Before the mid 1960’s, all the wheel brakes were fed brake fluid through a single system. If one leak developed, all the braking was lost. Vehicles now have dual braking systems in one master cylinder, where each system supplies fluid to a pair of wheels. A leak on this “dual” system will still allow at least two of the vehicle’s wheels to be braked. Braking performance will be decreased significantly but at least you will be able to stop. A brake pedal that pushes most of the way to the floor is an indication of a fluid leak. A red brake warning light should also turn on to indicate a problem that needs to be looked at immediately.
Worn brakes can also cause a low brake pedal. Disc brakes are self-adjusting, but the drum brakes on many makes of vehicles only self adjust if you step on the brake pedal while driving backward, or on others if you use the parking brake. Many drivers seldom back up and never use the parking brake, so their drum brakes would be out of adjustment. Drum brakes can be manually adjusted, but you can also do it yourself with the proper driving methods.
A visual inspection of brake pad thickness is usually performed to determine their serviceability. If the lining material is less than the thickness of a dime, the pads or shoes need replacing NOW! If allowed to wear completely through to the metal backing, the rotor will be destroyed in a few stops. This is also accompanied by loud grinding sounds as you step on the brakes.
Brake squeal is caused by the rapid vibration of the brake pad against the rotor and is most annoying under light brake applications. Different types of brake pad material make different noises, with softer organic materials making less noise than more metallic compounds, but all make noise. Dust seems to aggravate the problem. Cleaning the brakes and applying an anti-vibration compound to the back of the brake pads will work for a while, but some brake noise is common with all vehicles. Bedding in the brake pads by making about 10 medium-hard stops from about 50 km/h with a one minute cool down period in between will help minimize brake noise. The pad material actually melts onto the rotor surface in a fine film to provide better friction for stopping.
Warped brake rotors most often cause pedal pulsation or vibration. On some cars, rotors warped as little as .25 mm will cause a problem over time. Brake rotors can be warped by overheating them such as riding the brakes down a long downhill or applying a parking brake immediately after performing some fast stops. Most of the time warped rotors are caused by unevenly torqued wheel nuts or dirt trapped between the rotor and its mounting surface or the wheel. Cleaning the back of wheels and rotor mounting surfaces when you change tires will help prevent warped rotors. If you already feel pedal pulsation in your car, then the rotors need to be machined true again to correct the problem.