Understanding Dodge Motorhome Brakes
This article appeared in an older newsletter when there were a lot of Dodge Motorhomes in The Family Motor Coach Association.
I find it very informative.
James Gary "Andy" Anderson
A GUIDE TO MOTORHOME BRAKES
Understanding the basic operating principles of this
important coach system can help coach owners to keep
their brakes in optimum working order.
By C. Jay Haynor,
Editor’s note: The purpose of this article is to familiarize the reader with the braking systems used on most Class A and Class C motorhomes. Discussions of unique brake issues will also be included. Air brake systems will not be covered at this time.
Vehicle braking systems have undergone an interesting evolution, starting with wooden “brake shoes” and advancing to today’s four-wheel antilock brakes. A good beginning reference point is the 1906 Ford Model K. It had a 6-cylinder engine rated at 40 horsepower and was capable of the then astonishing speed of 50 miles per hour. The fuel tank had a capacity of 15 gallons, giving the vehicle a driving range of 250 miles. To stop the car, the driver pulled a lever to apply a mechanically controlled hub brake. If there was an emergency or a malfunction with the service brake, a pedal-operated emergency brake could be employed by stepping down to apply the brake on the drive shaft. This is the exact opposite of most braking systems today, which have a foot-applied service brake and a hand-applied parking brake. Today’s vehicles also have split brakes, “X” brakes, disc brakes, hydro-boost systems, power vacuum systems, and so forth.
There are actually two braking systems on a motorhome chassis as it leaves the assembly line: a service brake system and a parking brake system. The service brake system consists of all the components that allow the driver to reduce or halt, directly or indirectly, the speed of a vehicle during normal driving. The action is gradual. The parking brake system consists of all the elements required to hold the vehicle stationary mechanically even on an inclined surface, and particularly in the absence of the driver.
The hydraulic braking systems used on motorhomes slow down or stop a vehicle by causing friction between a brake pad or brake shoe that does not rotate and a brake rotor or brake drum that revolves with the wheel. It is this friction that creates the force applied to slow a moving vehicle.
As one might expect, when the brakes are applied, the energy of the vehicle is changed into heat. An emergency stop at 60 miles per hour can cause the temperature of the brake linings to rise to in excess of 450 degrees Fahrenheit on a car and in excess of 750 degrees Fahrenheit on a motorhome. A delicate balance must be struck in the design of the braking system: pads and linings must maintain enough friction to provide adequate braking force, but not so much friction that the linings burn out.
The friction is measured as a “coefficient.” The friction coefficient of the linings must stay within narrow limits over a wide range of temperatures for satisfactory performance. Carefully chosen friction materials are used by the chassis manufacturer in a motorhome braking system. One of the most important points I hope every reader derives from this article is that replacement brake pads and linings should be chosen carefully. Do not shop on price alone!
Currently two distinct lining compositions are used for disc brakes: semi-metallic and non-metallic. The entire braking system is designed around a particular composition. It is very important that the same composition, semi-metallic or non-metallic, be used when replacing disc brake pads. Non-metallic brake linings polish the rotor surface smooth as they are applied, while semi-metallic linings deposit some of the metallic material on the rotor surface during brake-in. If you were to look at a rotor of a braking system that uses semi-metallic brake linings, you would see a dull rotor surface. In this system you have a brake rotor and brake lining that are matched, allowing the semi-metallic lining to “grab” properly. Generally, semi-metallic linings progressively become more effective with use, especially in cold weather, and they tolerate more heat. Non-metallic linings usually begin to fade after multiple hard applications.
If you install non-metallic linings on a brake system that was designed to use semi-metallic linings, the brakes will not work properly initially. The linings will have to wear the metallic deposits off of the rotor before they function correctly. The result will be premature brake lining wear. On the other hand, if you use semi-metallic linings on a system that was designed to use non-metallic linings, the pores in the brake rotor that would fill with metallic deposits instead will be filled with organic material, preventing metallic deposit transfer.
Another point is that if a brake rotor surface has been machined in a system equipped with semi-metallic brake linings, a break-in period is required to build up adequate material transfer. Normally this occurs after approxi- mately 50 miles of stop-and-go driving, during which hard braking should be avoided.
Hydraulic braking systems on motorhomes utilize either a vacuum booster or a hydro-boost system. We will first discuss the vacuum booster system, which generally is used on older coaches.
Hydraulic brakes with a vacuum booster. Braking begins with the driver’s foot depressing a brake pedal. The mechanical force is transmitted into hydraulic pressure inside a master cylinder. The hydraulic principle states that hydraulic fluid is virtually incompressible. Pressure applied to the piston of one cylinder can therefore be transmitted by the hydraulic fluid through lines to a second piston. The force applied to the second piston is proportional to its surface area. If the second piston is the same size as the first, the force applied to the second piston will be equal. The second piston will also move the same distance as the first. If the second piston has a larger surface area than the first, the force applied with be proportionally greater; however the second piston will move a shorter distance. At times technicians will attempt to change the braking balance from the front brakes to the rear brakes by changing the rear wheel cylinders to a larger piston size. This should not be done unless the chassis manufacturer issues a service bulletin. Brake systems are designed and tested under many different circumstances and should not be altered.
Between the brake pedal rod and the master cylinder is a sealed, round chamber called a vacuum booster. Another rod comes out of the vacuum booster into the master cylinder. Inside the booster is a diaphragm. On the master cylinder side of the diaphragm, a large vacuum hose goes through a one-way check valve and is sourced b y the engine. (Diesel engines are an exception, as they do not produce vacuum; diesel engines require an external vacuum source such as a belt-driven or gear-driven vacuum pump.) Some systems have a can in the routing that serves as a vacuum storage tank. When the engine is running and the brakes are applied, vacuum created on one side of a diaphragm and atmospheric pressure pushing on the other side greatly reduces the pedal effort on the part of the driver as compared to a system with non-power-assisted brakes.
To test the vacuum booster, pump the brakes several times to expel any vacuum reserve. Place your foot on the brake pedal and start the engine. The pedal should be very high; however, within a few seconds, as vacuum builds up, the pedal should go down to the normal position. If the pedal acts otherwise, brake system inspection is in order.
Some of the 1976-1981 Dodge M500 and M600 chassis braking systems required high pedal effort to stop the motorhomes. The “fix” was to install a second vacuum booster in tandem with the existing booster. One source of a kit to correct this condition is Big John’s, 7917 Seventh St., Downey, CA 90241; (310) 869-6298.
Since the late 1960s the master cylinders used have been dual cylinder units. One push rod drives two pistons., Between the dual pistons are seals that separate the master cylinder into two chambers. In the case of a motorhome, one chamber controls the front brakes, and one chamber controls the rear brakes. The systems function independently; so, if a hydraulic failure occurs in one system, the other will still operate. It should be noted that the brake pedal will be substantially lower if a failure in one system occurs. Also, in the case of most dual brake systems, a light on the dash illuminates if a difference in pressure between the front and rear brakes is detected.
The fluid used in braking systems is very special and must meet specific requirements:
- It must be very stable and boil only at a very high acceptable temperature.
- It must be equally viscous at all temperatures and not increase in viscosity excessively at low temperatures, and its freezing point must be low enough to ensure that it will function under the worst conditions likely to be experienced.
- The fluid must have lubricating qualities.
- It must not attack any of the materials with which it is likely to come into contact.
- It must inhibit corrosion.
Several groups, including the Society of Automotive Engineers (SAE), and the federal government establish the requirements for brake fluids. The Department of Transportation (DOT) has defined specific boiling points for brake fluids. DOT 3 brake fluid is used by most chassis manufacturers and has a minimum boiling point of 401 degrees Fahrenheit. DOT 4 has a minimum boiling point of 446 degrees Fahrenheit. DOT 5 brake fluid is a silicone-based brake fluid and has a minimum boiling point of 500 degrees Fahrenheit. While on the surface it may appear that the silicone-based DOT 5 fluid is the best choice, several other factors must be considered. For one, DOT5 is more compressible than DOT 3, and a “soft” pedal feel may be experienced when using the former. In addition, unlike DOT 3 fluids, silicone brake fluids do not absorb moisture. The result is that if enough moisture is present, pockets of water form. These pockets could migrate to low points in the system, such as the wheel cylinders or calipers. At 212 degrees Fahrenheit the water will boil, and at 32 degrees Fahrenheit the water will freeze. Neither Chevrolet nor Ford recommends the use of DOT 5 fluids; both specify either DOT3 or DOT 4 brake fluid. Some DOT 3 fluids have a boiling point in excess of DOT 5 and as high as 550 degrees Fahrenheit.
Brake fluid should be purchased in a sealed container, such as a can that can be resealed after the cap or lid has been removed originally. If only a small amount of fluid remains in the can after you have opened and used it, discard the can. The humidity in the air that enters the can will reduce the boiling temperature of the fluid. This is another area where cost is not the most important issue.
One question that is asked bout brake fluid is should it be replaced, and if so, how often? The answer is an unequivocal “YES.” Your reply to this might be, “Why?” and “Why isn’t brake fluid replacement part of maintenance listed in the owners manual?” – two very valid questions. Over time brake fluid absorbs small amounts of water, and this lowers the boiling point of the brake fluid. In a motorhome the boiling point of the brake fluid is critical, especially if the coach tows another vehicle or is used in hilly or mountainous areas. In response to the question about brake fluid replacement being part of the maintenance schedule, it is for some European cars, and I feel confident that the American automobile manufacturers are looking at periodic brake fluid replacement and will soon include it in the owners manuals. I’m sure concerns exist regarding the fact that the procedure must be performed properly, new sealed brake fluid must be used, and the proper equipment for brake bleeding, as discussed elsewhere, must be employed. If not, the braking system could be less effective than before the service was performed.
It should be noted that if DOT 5 silicone brake fluids used, the change interval should be shorter than when DOT 3 or DOT 4 is used; in fact, some technical people recommend changing silicone brake fluid every six months. The reason is that unlike DOT 3 and DOT 4 fluids, the water that accumulates in silicone brake fluid will not mix and could create a gap or steam bubble in the system. This could cause a substantially lower brake pedal than normal.
Testers that can measure the boiling point of the brake fluid in a particular system just by taking a sample from the master cylinder are available. One such system is available from Schroeder-Nielsen Recovery, 2615 River Road, Unit 3, P.O. Box 2366, Cinnaminson, NJ 08077-5366; phone (609) 829-9144, fax (609) 829-8814. This particular system simply connects to the vehicle battery while a probe is placed in the master cylinder reservoir. A heater button is pressed and within a minute the boiling point of the fluid is displayed, indicating the brake fluid’s safety level. The tester is expensive – approximately $400 – so it is unlikely that you will want to “order a few today”; however, a reputable service facility that performs brake service should consider having one.
Manfred Dickersbach, operations manager of Schroeder-Nielsen, advises that used brake fluid has been considered a hazardous waste since April 1993 and should be disposed of accordingly.
Brake fluid leaves the mater cylinder through steel lines. These lines my change to special rubber fabric high-pressure lines before the lines reach the wheel cylinders or calipers. Do not substitute materials for any brake line; use the same material as originally installed by the manufacturer.
Three types of braking systems can be installed at the wheels: drum brakes at all wheels, front disc brakes and rear drum brakes, and disc brakes at all wheels. Motorhome chassis use all three systems. Early motorhome chassis, generally before the 1973 models, used four-wheel drum brakes. In mid-1973 Dodge introduced front disc brakes on its Class A chassis. Dodge never did have four-wheel disc brakes on its motorhome chassis, but the popular Chevrolet Class A does.
Drum brakes are actuated by a wheel cylinder that normally has two pistons. The brake fluid moves the pistons into two piston pins directly attached to the brake shoes. The brake shoes are anchored at the bottom and move out against a brake drum. One key area should be watched. The brake drum surface area must be kept smooth. If the brake shoes wear too far, the surface area of the drum can score and then must be machined using a brake lathe. There is a maximum diameter to which the brake drum can be machined, and this number usually is stamped into the brake drum. Do not exceed this diameter. The brake drum dissipates the heat caused by the friction that results when the driver uses the brakes. The more metal there is on the brake drum, the better the heat dissipation. If you exceed the brake drum diameter, the brake drum will not have enough metal to adequately dissipate the heat, and brake fade could occur.
The disc brake calipers used on motorhome chassis are a floating design; the entire caliper moves, as do the piston housed inside the calipers. A flat pancake device called a brake rotor is straddled by a caliper that is held in place by an anchor plate and a front wheel spindle. Generally there are two large pistons inside the caliper on the same side working in tandem. These pistons are much larger than brake shoe wheel cylinder pistons, because unlike drum brakes, where frictional drag pulls the shoe into the drum, disc brakes are perpendicular to the rotor, requiring much more hydraulic pressure to produce the same results. The inner disc brake pad attaches to the pistons. The outer disc brake pad attaches to the caliper housing on the other side of the rotor. Hydraulic pressure from the master cylinder applies pressure to the pistons. This moves the inner brake pad into contact with the rotor surface. The reaction of the caliper away from the piston also forces the outer pad into contact with the rotor, in a “clamping” action. Note: vehicles equipped with a floating caliper disc brake system do not have a “rock hard” pedal as prior systems did. The pedal appears to fade as you apply more pressure while standing still; however, the vehicle will not move. This is a normal operating characteristic of this system. It is not normal, however, for the pedal to go to the floor and the brakes to be ineffective.
There are two different disc brake piston compositions: metal and phenolic. Phenolic pistons are made of resin reinforced with glass fiber. In addition to weighing less than steel, they provide much greater insulation against heat. This is very critical in motorhome operation. Under certain circumstances, such as going down steep hills, the disc brake pad can become very hot – in excess of 750 degrees Fahrenheit. The pad is contacted by the piston, which is applied by brake fluid under pressure. Heat transfer can occur back to the brake fluid. A steel piston can reach a temperature of 700 degrees Fahrenheit. Remember, DOT 5 boils at 500 degrees minimum, and some DOT 3 fluids boil at 550 degrees Fahrenheit. This, however, can still fall short of our requirement, possibly allowing brake fluid to the front brakes to boil. The phenolic pistons have one very distinct advantage: they provide insulation to prevent heat transfer to the brake fluid. You should consider replacing the pistons on your motorhome disc brake calipers with phenolic pistons if you drive in areas that require heavy braking.
When phenolic pistons first entered the market, there were some instances of premature brake pad wear thought to be caused by the phenolic pistons binding in the caliper bore. This gave the product a “bad name.” Consequently, most technicians resorted to replacing the phenolic pistons with traditional steel pistons. I feel that most of this was due to the repair industry resisting change.
The topic of disc brake noises should be discussed at this time. Some brake noises are normal, and some are not. The most common noise associated with disc brakes is a squeal, usually occurring upon light braking. This is especially noticeable when driving in the left-hand lane of a road divided by concrete barriers, because sound bounces off of these walls. The noise is the result of disc brake dust buildup, which is not detrimental and does not affect the integrity of the brake system, but it can be irritating to the driver to the point of embarrassment. Notice that the front wheels of cars with front disc brakes are always dirty. This is the result of disc brake dust that is created when the driver applies the brakes. As the dust builds up, the squeal intensifies. Often a few hard brake applications will temporarily eliminate or reduce the squeal. A brake task force from the Society of Automotive Engineers has been addressing this issue for a number of years. The task force has not yet issued its first recommendation.
Yes, there are many chemicals on the market that address this condition. Some are applied to the back metal portion of the disc brake pad to absorb the squeal. These chemicals sometimes are moderately effective for a short period. Some technicians have altered the disc brake pads to allow the dust to accumulate in the void area. Again, this is a short-term solution, and it should not be done, as it can affect the integrity of the braking system. A few chemicals on the market are intended to be applied to the actual disc brake lining to saturate it. Under no circumstances should these products be used, as they will definitely affect brake lining integrity.
Some disc brake systems have wear indicators on one of the pads, usually the outer one. The wear indicator is a thin metal strip that will make contact with the brake rotor before the lining is worn through and the metal portion of the pad makes contact with the rotor, necessitating rotor matching or expensive replacement. When the wear indicator makes contact with the rotor, a loud squeal is heard, indicating that brake inspection is in order, and the necessary services should be performed.
A great deal of confusion exists as to how disc brakes “self adjust.” Some people think it is done on the caliper slide anchors, but this is not accurate. Inside the caliper housing piston bore is a slot for a square-cut “O” ring. This “O” ring serves four purposes:
- It produces a seal between the piston and caliper, preventing brake fluid from leaking.
- It functions as a “return spring” for the piston.
- It becomes distorted, allowing the disc brake pad to come in contact with the rotor when the pedal is applied.
- As the disc brake pads wear, the “O” ring allows the piston to move out farther and self adjust.
Our discussion does not end here. When a Coach comes to a stop, a significant weight transfer from the rear to the front occurs, In vechicles with front disc and rear drum brakes another two valves are required to equalize braking – a proportioning valve and a metering valve. These valves can be separate or in combination as one valve.
The proportioning valve distributes outlet pressure to the rear brakes after a predetermined rear input pressure is reached. This is done to prevent rear wheel lockup.
The metering valve limits the pressure to the front disc brakes until a pressure high enough to overcome the rear shoe retracting springs is reached.
On some Ford and Chevrolet Class C chassis, a height-sensing valve is used to fine-tune the braking balance. The valve is mounted on the frame above the rear axle. A rod is connected to the rear axle. This rod moves an internal valve to regulate the brake pressure to the rear brakes. As weight is added to the rear, the distance between the frame and the axle decreases, necessitating more brake pressure to the rear. As weight is subtracted, the opposite takes place.
Most chassis have a brake light on the dash. Some people assume that when they see a brake light lit on the dash it indicates that the parking brake is applied. While one light may provide this type of warning, another brake light illuminates when the ignition key is in the “start” position and then normally goes out. If this light comes on while the motorhome chassis brakes are being applied, it indicates that a pressure difference exists between the front and rear brakes. The result will be decreased braking efficiency and a lower brake pedal. Immediate brake system attention is required.
You ask how and why the brake light goes on when the ignition key is in the start position? When the key is in the start or run position, the brake circuit is receiving 12 volts. What is needed is a ground, which is supplied when the key is in the start position, thereby completing the circuit. This is called “ground side switching,” and it provides a “prove out” to determine that the system and light are operational.
Hydraulic brakes with a hydro-boost system. Now that we have discussed the vacuum booster system, let’s discuss an alternative to the vacuum booster, called the hydro-boost system. It is used on most Chevrolet and Ford Class A chassis and some others. The only difference between the hydro-boost system and the vacuum boost system is that the former incorporates a device that uses power assistance from the power steering pump instead of a vacuum booster. All other components in the system are the same.
Instead of using engine vacuum to assist brake effort, as the vacuum booster does, the hydro-boost uses the power steering pump to create a pressure within the assembly. A spool valve with the assembly controls how much pump pressure is supplied during braking. Should the power steering pump be disabled, an electric pump and accumulator will supply minimal assist.
Both Chevrolet and Ford shop manuals explain that under some unusual circumstances strange noises can be heard when a hydro-boost system is used. One of the most common circumstances is when excessive pedal effort is applied after the vehicle has completely stopped and the engine is still running. Another is when the steering wheel is turned from side to side while the vehicle is completely stopped and the brake pedal is applied. Under these conditions a hiss or a moan is completely normal. My analysis is that if you do abnormal thing such as those mentioned above, you will experience abnormal results or side effects.
When pumping the brake pedal of a coach equipped with a full drum brake system causes the pedal to rise, it is an indication that air is in the system. This air must be removed by bleeding the system in one of two ways. One is to either use a power brake bleeder or to pump the brakes the pressurize the system and then looses a bleeder screw to purge the air. In recent years vacuum bleeding has been found to be very effective. The vacuum source is attached, using a clear hose, to each bleeder, which is opened, until all of the air is purged from that line. On a brake system with floating caliper disc brakes, the pedal will raise slightly as it is pumped as a result of the pistons moving out.
Parking brake systems. The parking brake system on Class C motorhomes consists of a parking brake apply mechanism, a cable, an equalizer, and two cables – one going to the left rear wheel service brake and the other going to the right rear wheel service brake. Hint: many times when the parking brake goes to, or almost to, the bottom of its travels, it is an indication that the shoes are worn down and should be inspected and possible replaced. This is especially true in a Class C motorhome. The reason is that although the rear drum brakes are self-adjusting, they self-adjust only as the vehicle is backed up and stopped, and backing up in a motorhome is not all that common.
The other type of parking brake system normally is found on Class A motorhomes with four-wheel disc brakes. The parking brake system is mounted on the rear of the transmission. It is a drum brake system similar to that used on wheels. The Ford system utilizes a shaft housing, containing two bearings in a fluid bath, that attaches to the rear of the transmission. The fluid should be checked periodically and filled to the bottom of the fill hole. This is something that isn’t normally done, because many technicians are not aware of these components. However, neglecting to do so could cause bearing failure.
The parking brake system consists of brake shoes that move against a brake drum connected to the drive shaft when activated by a cable. In effect, the parking brake prevents the drive shaft from rotating. If you have this type of system, be very careful when jacking up one side of the rear of the vehicle. BLOCK THE WHEEL OPPOSITE THE ONE YOU ARE REMOVING.
Antilock brakes. I would like to take a moment to briefly mention antilock braking systems (ABSS) and discuss a myth commonly associated with them. The sole function of ABS is to prevent wheel lockup during braking, allowing the vehicle to stop straight. This is done either by a computer pulsating wheel braking or a computer regulating pressure to the necessary wheel(s). Some people have reported that ABS braking systems require longer stopping distances. First of all, the antilock portion of the braking system works only when the computer senses that the affected wheels are not stopping equally. The ABS system does not become involved under normal circumstances, so braking remains unaffected.
When conditions are such that the antilock portion of the braking system is activated, braking distance probably will be increased in comparison with the stopping distance on normal, dry pavement. Unfortunately, this is not a valid comparison. The alternative to antilock is uncontrolled braking, simply defined as skidding. With any new technology, effective communication regarding the change is necessary. In some instances the people who speak negatively about ABS may not understand how the system works. Unfortunately, the industries involved with antilock braking systems have not done a good job of communicating. The effective communication is just now beginning.
Each year I drive many different vehicles under all circumstances, logging many thousands of miles. Having used antilock brakes many times both intentionally and unintentionally, I can tell you that in every instance they have performed as designed, allowing me to stop straight and safely, and preventing accidents on a few occasions. My personal experience ranks antilock brakes as one of the top safety features of all time. Should you be purchasing a vehicle that has ABS as an option, I would suggest that you give them serious consideration.
Brakes rank among the most important components on a motorhome, and gaining an understanding of their basic workings can help motorhome owners to guard their system by making sure that the system is working as it should.