The comedian George Carlin used to say, “When you step on the brakes, your life is in your foot’s hands.” How true this statement is, but there is much more than meets the eye (or should I say foot?) when it comes to the braking system in a vehicle. Arguably one of the most critical components on a vehicle is the braking system, yet it can also be the most neglected. In this article I will discuss the components which make up a diesel pickup’s braking system and the proper maintenance of the vehicle’s brake components.
The components involved in a common diesel pickup’s braking system includes—but is not limited to—the master cylinder, brake booster, brake fluid, brake lines, anti-lock brake distribution block and ABS module, all of which are located under the hood. Along the frame and at the wheels include components such as wheel end cylinders (calipers), wheel speed sensors, brake pads and brake rotors. Each component plays a very vital role with the vehicle’s braking ability. These components will be discussed in regards to the principles which make them work, their proper maintenance and diagnosis of failure in the particular component.
Let’s start at the top with the master cylinder. The master cylinder is mounted on the back side of the bulkhead or firewall and is the component which is tied indirectly to the brake pedal inside of the cab. As you put pressure on the brake pedal it pushes on a cylinder or piston in the master cylinder, which directs brake fluid, under pressure, eventually to the brake calipers. This direction of fluid usually is broken up into two separate circuits, hence we see two separate lines come out of the cylinder. Master cylinders usually have the brake fluid reservoirs mounted directly on top of them. Upon closer inspection of the brake fluid reservoir, one will discover there are again two separate partitions in the reservoir—one for each circuit. This is for safety purposes. Federal motor vehicle safety standard (FMVSS 105) mandated in 1976 that vehicles have dual brake circuits. The theory was if a brake line bursts in the system and pressure is lost there is still another half of the system, which can enable the vehicle to safely stop.
The brake fluid plays a very vital role in the system’s function. The following brake fluid types will be discussed: Dot 3, Dot 4, Dot 5 and DOT 5.1. All brake fluids must comply with FMVSS 116. Braking systems on light trucks and passenger cars will use DOT 3 brake fluid more than 90 percent of the time. Dot 3 is very common and is a hydraulic fluid which is hygroscopic, meaning it can absorb moisture. It also has a relatively high boiling point—400 degrees Fahrenheit when dry and about 280 degrees when it has absorbed moisture. These two ratings are called wet and dry boiling points. DOT 4 brake fluid is very similar to DOT 3, but has a higher boiling point: 440 dry and 310 wet. Dot 5 is not similar to DOT 3 or 4. It is not hygroscopic, meaning it does not absorb moisture. It does have a very high boiling point: 500 dry and 350 wet. DOT 3 can mix with DOT 4 if upgrading fluids, but DOT 5 cannot mix with 3 or 4. Due to this problem the industry came up with DOT 5.1, which will mix with DOT 3 and Dot 4 but has very similar boiling point ratings as high as DOT 5.1 fluid. Brake components can get very hot when utilizing the brakes often so the possibility of the brake fluid reaching boiling temperature is likely. When brake fluid boils the effectiveness of the brakes diminishes very rapidly. When excessive city driving takes place or towing, the need for higher rated brake fluid occurs.
It still seems amazing to think that the power of your leg has the ability to stop a 10,000-pound pickup with ease. Well it really doesn’t. Your leg acts as the force which begins to push on the master cylinder’s piston, but the real force comes from the brake booster. There are several different kinds of brake boosters, but the vacuum booster and hydro booster are the two most common. For this article I will primarily focus on the hydro booster, since it is found on most diesel pickups due to the lack of vacuum developed by the diesel engine and power needed for heavy applications. Brake boosters are located in-between the master cylinder and the brake pedal. As you apply force to the brake pedal you are also pushing on internal valving in the hydraulic brake booster. The brake booster utilizes power-steering pressure via hydraulic lines from the power-steering pump. If you never thought your power-steering could affect your brakes, think again. The next time you step on the brakes and hear your power-steering pump whine, you will know why. As you push on the brakes you are still in complete control but the hydro-booster “assists” or “boosts” your leg in braking force applied to the calipers. The vacuum booster works on the same principle as the hydro-booster, but utilizes vacuum to pull against a large rubber diaphragm instead of a hydraulic piston found in the hydro boost.
As brake fluid under pressure leaves the master cylinder it is directed to the abs valve module if the vehicle is equipped with anti-lock brakes. Anti-lock braking systems can range from the very simple 1-sensor, 1-channel systems found on rear anti-lock brakes to the extreme 4-sensor, 4-channel found on four-wheel anti-lock brake systems. In a future article the details of ABS may be discussed, but due to their depth and complexity it will be saved for later. Simply put, Anti-lock braking systems greatly reduce stopping distances in very slick road conditions when a skidding situation may be present. As fluid passes through the ABS distribution block it travels down rubber and steel lines and ends up going into the wheel cylinders or calipers. Calipers come in a variety of sizes and can have one or more pistons arranged on one side or both sides of the caliper frame. As brake force is applied, fluid fills the piston(s) found in the calipers and the pistons extend out. Once the pistons extend they begin to exert force against the brake pads, in turn pushing the pads into the rotor and creating friction.
A very well-known belief in physics is the fact that energy is neither made nor destroyed—it is just changed. The inertia or rotating energy we have from our 10,000-pound truck moving at any speed when trying to stop is then turned into another form of energy—this energy is heat. The friction from the brake pads rubbing against the disc brakes if so desired will stop the vehicle. Seems simple, just movement of fluid back and forth and friction to stop the vehicle. It is simple, but the system still requires routine maintenance to prevent failure.
The best place to start is the brake fluid. It is not uncommon to pop the hood on a pickup which is 10 years old (or older) and the brake fluid has never been changed. The brake fluid which at one time was clear now looks similar to black engine oil. The darker the fluid the more moisture it has absorbed over the years. More moisture means a lower boiling point. As stated earlier when brake fluid boils, the brakes begin to have a real “spongy feeling” almost as if air is in the system. Routine brake flushes should occur about every 30,000 to 45,000 miles. More than one ABS distribution block has been ruined due to improper maintenance of the brake fluid. One thing we learn about moisture against internal metal parts is that it can create oxidation and corrosion. Prematurely failing master cylinders and sticking brake calipers often times are a result of moisture-saturated brake fluid. Another component which requires routine maintenance which is associated with the master cylinder is the brake booster. If your diesel pickup is equipped with a hydro booster then it is not a bad idea to flush the power-steering system every 60,000 to 75,000 miles to assure proper valve actuation in the hydro booster.
Another area of maintenance undoubtedly is the brake pads themselves. Brake pads should be checked every time the engine oil is changed. Some brake pads can last over 100,000 miles, while different driving conditions on the same vehicle or failing calipers can constitute a pad change every 15,000 miles. Legally the pads on any light duty vehicle need to be over 1/16 inch according to the department of motor vehicles out of service criteria. Most pads will indicate to you as the driver by “squealing” before they wear to the minimum thickness of 1/16 inch. Wear indicators are designed to scrape the rotor once the pad has worn to roughly 1/8 inch. If the vehicle’s pads are neglected and worn past the minimum then the pad’s metal backing plate will contact the rotor. This always calls for the rotors to be machined or replaced.
The last area which will be discussed in terms of maintenance will be that of the brake rotors. Rotors are the component which is linked directly to the lug bolts and wheel. They are usually made out of very high ductile cast iron and can withstand high temperatures….. to a degree. When too high of a temperature is sustained from excessive braking, then warpage can occur. Warpage is when the metal is distorted due to high temperature expansion rates and never returns to its original shape. Warpage can be found in terms of rotor runout and width variation. Both conditions often yield vibration when braking in the vehicle and require that the rotors be machined or replaced. Aggressive braking, riding the brakes on steep inclines with a load, and improperly vented rotors can all lead to warpage.
Several components can and do contribute to effective braking on modern diesel pickups. When several components are involved on any vehicle’s systems and subsystems, maintenance is always a key to making them last. So the next time you fly up to an intersection at a red light and gently touch that rubber covered foot rest you call a brake pedal, you will appreciate the sensation of stopping a little more.
About The Author
Looking to learn more about diesel systems? Want to make a career out of building, repairing and installing performance components? Levi Perkins is the head instructor for the diesel program at the College of Southern Idaho located in Twin Falls, Idaho. As a student of the diesel program at CSI, you will also participate as a member of the Diesel Club, in which you'll be a member of various fundraisers, participate in the World Agriculture Expo, tractor pulling events and many other educational activities.
Classes are held from 8:00 a.m. to 1:00 p.m. Monday through Friday, and will give students the required knowledge and practical application experience for diesel systems, which include: engines, transmissions, axles, brakes, electrical and hydraulic systems and much more. The diesel program at CSI is proud to announce they've had a 100 percent placement in the last three years and look forward to giving future diesel mechanics and engineers a bright future in the diesel industry of tomorrow. For more information about the diesel program at the College of Southern Idaho, go to www.csi.edu or call 208-733-9554.