What you need to know about your motorhome braking system to assure a safe stop every time.
By Peter D. DuPre
Even though it happened more than a decade ago, the memory is still fresh. With our motorhome’s fuel tanks full, food loaded, and the family aboard, I headed out of a Santa Clarita, California, fuel stop. For a few miles, at least, everything went as planned. We aced the climb up the state’s infamous Grapevine Hill on Interstate 5, known for its extremely steep grades, with nary a move of the cooling gauge needle, even though outside temperatures were pushing 90 degrees. We breezed through Gorman and started down the back side of the “Vine,” heading toward Kings Canyon National Park.
As the vehicle picked up speed on the downhill stretch, I gently applied a little brake pressure to slow it down and shifted the transmission down a gear. As the hill grew steeper, I used the pedal more and more. Suddenly, I pressed the pedal and got an eerie feeling in the pit of my stomach as my foot pushed the pedal to the floor. I pumped the pedal, hoping the pressure would come back up. It did to an extent, but not enough to significantly slow down the RV. The motorhome began picking up speed, and even another downshift left me traveling too fast. Luckily, the highway has deep runaway ramps at regular intervals down the hillside. I headed off the freeway and onto one of the runaway ramps, where 6-inch-deep loose gravel allowed me to bring the coach to a safe and secure stop.
It turned out that the problem was a burst brake line. The rubber had dried out, cracked, and ruptured under pressure. At first, I was more than a little surprised that my motorhome had experienced a brake failure. After all, I kept it in pretty good mechanical condition, and I had checked the brake fluid level only that morning.
Motorhomes, it seems, don’t do well when left unused for long periods of time. Hoses rot, moisture creeps into vital mechanicals, parts rust, fluids evaporate, batteries discharge, seals dry out, and a host of other problems can occur just from it sitting in a storage lot. Many of these problems are discovered during pretrip inspections by owners and/or their mechanics and quickly can be repaired. Brake systems, however, are often neglected save for a semi-regular fluid check and a quick pump or two of the pedal. One problem is that except for the pedal and the fluid reservoir, most of the brake system is positioned in hard-to-see and difficult-to-reach locations. Also, few motorhomes accrue enough regular mileage to where brake jobs are an everyday occurrence. Just as your family car can go 40,000 miles or so before needing new brakes, so can most RVs. This means they don’t get checked often enough. In fact, many RV owners may not even know exactly what type of braking system they have. Sure, they may know that the coach has “disc” or “air” brakes, but do they know exactly what those terms mean or how the systems work?
Four basic types of braking systems currently are used on RVs: power-boosted hydraulic drum brakes; power-boosted hydraulic disc brakes; hybrid power drum/disc brakes; and air brakes. The system in use in your motorhome will depend upon the vehicle’s age, type (type A, B, or C), and manufacturer. In this article, we will look at the different brake systems, discuss some common problems, and provide some brake system maintenance tips. We will not be covering exhaust or engine brakes, as these do not work directly on the wheels but utilize engine compression to assist braking. That’s a whole different article.
Before learning about the differences among the various braking systems, it is best to understand how the basic brake system works. As you know, the purpose of the brake system is to bring a rolling vehicle to a safe stop. It does this by converting kinetic energy, the energy of motion or movement, into heat energy. The heat is generated by the friction of the pads or shoes pressing against the rotors or drums to slow the spinning wheels of the vehicle. This energy conversion begins when your foot presses on the brake pedal.
On a hydraulic system, the pedal is attached to a shaft that connects through a vacuum-assist unit to the master cylinder. It is the ratio between the pedal length and the shaft pivot length that turns your 50 pounds or so of foot pressure into the several hundred pounds of pressure pushing on the vacuum diaphragm. This in turn increases the amount of force pressing on the pistons in the master cylinder, which pushes hydraulic fluid through the braking circuits.
It is really a very simple hydraulic principle: force exerted on the master cylinder pistons from pushing on the brake pedal moves the brake fluid from the master cylinder into the various brake lines. As these lines are already filled with fluid, pumping more fluid into them forces some of it out the opposite end of the line and into the wheel cylinder (drum brakes) or caliper reservoir (disc brakes), where the fluid forces out a piston that pushes on the brake shoes or pads, which in turn snug up against the inside of the wheel drums or against the rotors. The ensuing friction generates heat, which is dissipated into the atmosphere, and the wheel slows down. My description is a little oversimplified, but you get the idea.
Since we’ve mentioned that the system uses brake fluid, it seems fitting that a description of brake fluid be offered. What we call brake fluid is actually hydraulic fluid #151; a special liquid chemical compound that is used in brake systems to transfer force. This type of fluid is used because it is not easily compressible, and it can handle the heat loads generated during braking. The United States Department of Transportation (DOT) standardizes brake fluid and currently rates them as five different types: DOT 2, DOT 3, DOT 4, DOT 5.1, and DOT 5. Of these, DOT 2 is essentially straight castor oil and currently is not used in modern automotive applications.
The most popular fluids in today’s applications are DOT 3 and DOT 4. Most motorhome chassis manufacturers recommend the use of DOT 3. This polyglycol-based fluid has a brownish tint when new. This brake fluid absorbs moisture, which is both good and bad. It is good because by absorbing moisture in the system, water pockets that can cause corrosion are avoided. However, water absorption degrades overall performance of the fluid and lowers its boiling point. When any liquid boils, it lets off gas that is compressible, which can lead to brake fade. For this reason, it is important that brake fluid be changed annually (more often in severe conditions) and that the master cylinder be kept tightly closed. Moisture from the air is absorbed directly by the fluid, so keeping any spare fluid in a tightly sealed container is vital. If the fluid turns dark or black, it is a sign that it is contaminated with water and should be changed.
The accompanying chart on page 90 shows the difference in boiling points for DOT-rated brake fluids when dry (straight brake fluid) and wet (contaminated with water), giving you a clear idea of how much braking power can be degraded by excessive moisture absorption.
DOT rating Dry boiling point Wet boiling point
DOT 5 is a silicone-based hydraulic fluid that does not absorb moisture, which means that the boiling point remains relatively stable. But it also means that any moisture entering the system will collect in pockets. This may create corrosion within the system that could lead to brake system failure. It is recommended, therefore, that for RV use, DOT 5 be avoided. Also, DOT 5 cannot be used in any system with an antilock braking system (ABS). DOT 5.1 is a non-silicone formula that is compatible with DOT 3 and DOT 4 and is a reasonable substitute for motorhomes that see heavy brake use.
Most U.S. vehicle manufacturers currently use DOT 3 fluid in their braking systems, with DOT 4 considered for heavy-duty applications (including some motorhomes). The vehicle’s owners manual lists the type of fluid your motorhome requires. Make sure to use only the fluid type recommended by the manufacturer and do not mix different DOT-rated brake fluids. Some RV owners have been using DOT 4 fluid in a vehicle rated for DOT 3, and it generally is allowable, as there are no known problems in doing so. However, before swapping out brake fluid, make sure you check with an ASE-certified brake specialist AND the vehicle manufacturer.
Note: The DOT rating system requires that DOT 3, DOT 4, and DOT 5.1 be fully compatible and able to be mixed freely. However, it is not a good idea to mix “” unless forced to “” as the resulting mix will be no better than the lowest grade in the mix.
Finally, brake fluid is not friendly to painted and finished metal surfaces. When working with brake fluid, always take care to keep it from spilling and, just to make sure, rinse everything off with plenty of water afterward, even if you don’t think any fluid was spilled.
Drum and Disc Brakes
Although many newer RVs are equipped with four-wheel disc brakes, some units may have a combination system of front discs and rear drums, and older motorhomes may have only drum brakes.
While both hydraulic drum and disc brake systems operate in essentially the same way, a few differences exist. The first, and most notable, difference is in the frictional materials used and the way they interact with the wheels.
A drum brake consists of two large metal brake shoes covered with friction material that, when activated by the wheel cylinders, push out from the center of the wheel hub until they contact the inside surface of the wheel drum. This system is also designed to carry a small amount of pressure in the line even when the brakes aren’t being applied. System pressure is maintained by a check or a residual pressure valve in the master cylinder. A large spring in the master cylinder activates this valve so a light pressure is kept in the lines at all times to hold the wheel cylinder pistons out against the brake shoe tabs. This pressure is needed to keep the piston shafts from vibrating away from the shoes, which would cause a lack of brake pressure.
On disc brakes a check valve is not necessary, because the piston seal in the caliper assembly is much tighter, keeping the piston from vibrating away from the disc. Disc brakes do not use shoes and drums. Instead, a smooth metal rotor replaces the drum, and a caliper assembly with a large piston pushes two brake pads against the rotor from both sides to slow the spinning of the wheel.
Disc and drum combination systems are a hybrid of the two. In these systems, the front wheels use disc brakes, because the front wheels bear the greater braking force, and discs are more efficient. On these systems, a check valve is not used on the forward, or disc, circuit, but it is used on the rear drum circuit to keep pressure in the line.
Hydraulic Brake Problems
Brake drag. Brake drag can be caused by hanging calipers (primarily the result of corrosion or inadequate lubrication), air in the brake fluid, riding the brakes, linkage hang-up, and improper use of the brakes. Brake drag can overheat and damage the brakes.
Pedal goes to floor. This usually is caused by a loss of hydraulic fluid. Typically there will be some braking available just before the pedal hits the floor, and vigorous pumping may restore some level of braking to allow you to stop safely. If this happens, pull over; stop; and check the brake fluid level. You may be able to top it off enough to carefully drive to a mechanic. Realize that this is an emergency fix and that the vehicle needs immediate service.
Low pedal. A failed part in the self-adjusting hardware on drum brakes generally is the cause of this problem. If the distance between the shoe and the drum is not kept at the nominal dimension, the entire brake system compensates and causes the low pedal condition.
Spongy pedal. If the pedal feels “spongy,” it indicates that there is air in the system. When this happens, the pedal will go down almost toward the floor, but you usually will be able to “pump up” the brakes to get pressure into the lines. This is an unsafe operating condition and generally can be resolved by bleeding the brakes or purging the system of air.
Pulsating pedal. A pulsating or vibrating brake pedal can be caused by a variety of ills, such as loose wheel bearings, out-of-round rotors or drums, rust spots on the drums or rotors, or uneven surfaces. To determine where the pulsation is coming from, check whether you can feel the pulsing or vibration in the steering wheel when the brakes are applied. If you can, the problem is on the front wheels. If you can’t feel it in the steering wheel, the problem is in the rear wheels.
Hard pedal. This can be caused by a bad power booster or a loss of vacuum to the booster. On vehicles with disc brakes, seized caliper slides or pistons and/or pinched brake lines also can cause the problem. Again, a professional should check out the system and make the appropriate repair.
Brake fade. If you’ve ever noticed diminishing braking performance while going down a hill or in heavy traffic when the brake is applied multiple times over a short time period, then you know what brake fade is. As excessive braking action is applied over a short period, the pads, drums, rotors, and fluid heat up more quickly than ambient air can cool them down. Eventually the friction material on the shoes or pads begins to separate, the brake fluid starts to boil, and the pedal goes to the floor. If the situation becomes serious enough, a brake fire can occur. If you notice the onset of brake fade, pull over and park as soon as it is safe to do so. Let the brakes cool for half an hour or so before continuing. Then get the motorhome into a shop as soon as possible. The entire brake system may be compromised, requiring it to be rebuilt.
Grabbing. When one or more wheels lock up or grab when you barely touch the brake pedal, it means the friction material on one or more brake pads or shoes has been contaminated. This usually is caused by oil or oily chemicals soaking through the material, requiring the affected pad or shoe to be replaced.
Pulling. This most often is caused by a seized caliper piston or slide on disc brakes and a stuck or dragging shoe on drum brakes. A vehicle that pulls when the brakes are applied can cause a loss of vehicle control, because the driver tends to compensate by steering in the opposite direction of the pulling. On a slick surface or in a panic situation, the compensation factor becomes an extra hazard that can lead to an accident. Get the vehicle to a shop immediately.
Squealing. High-pitched squealing that occurs when the brakes are applied is almost always caused by the wear indicators on the disc brake rubbing against the rotor. Initially, the sound is a warning that new brake pads are needed, but if left unheeded, it also can indicate that the rivets holding the friction material on the pad mount are gouging grooves into the rotor. At the first sign of squealing, have the brakes checked.
Grinding. A grinding sound when the brakes are applied can mean that the brake shoes are worn down and that the mounting rivets are gouging out large grooves in the wheel drums. It also can indicate that gravel or grit is between the friction material and the drum or rotor. One other possibility is that a wheel bearing is going out. In any case, your brakes need the attention of a certified brake specialist.
Air brakes operate pretty much like any other brakes in that pressing on the pedal pushes the brake shoes or pads toward the drum or rotor to slow down the vehicle. The difference is that instead of using hydraulic fluid, the system uses compressed air to make the brakes function. Here’s how it works. Air is stored under pressure, or compressed, and when the brake pedal is pushed, air is forced out through tubes and hoses to the brake chambers located at each wheel set. When the compressed air enters the brake chamber, it pushes against a rubber diaphragm that, in turn, forces a steel push rod out to activate the brakes and slow down the vehicle.
While a good air brake system can offer superior stopping power for heavy coaches, one disadvantage is brake lag, which is the time it takes for compressed air to pass from the storage tank and through the system before braking actually begins. Hydraulic systems don’t have much problem with brake lag, because the brake lines are always filled with fluid and under pressure. But with air brakes, the lines do not hold pressure unless the brakes are being applied.
Once you’ve become accustomed to an air brake system, however, you should have no problems. And those who regularly drive many different coaches report no noticeable lag time in air-brake-equipped motorhomes built during the past few years.
An air brake system consists of three main subsystems: the compressor, tank, and lines that maintain the supply of compressed air; the valves that direct and control the flow of the compressed air; and, finally, the mechanical parts that transfer the energy from the compressed air into a mechanical force (the rubber diaphragm forcing out the steel rod) that activates the braking material.
The key to successful operation of an air brake system is to have a good air compressor and to be able to maintain proper air pressure in the system. Your air-brake-equipped motorhome will have one or more air pressure gauges to indicate the condition of the air system. Air compressors typically have a “cut-out” pressure between 110 psi and 130 psi, and a “cut-in” pressure about 20 psi lower. Air pressure buildup from 85 psi to 100 psi should take 45 seconds or less. Each time you start the coach after it’s been parked for a few hours or more, you’ll have to wait to move until sufficient and safe air pressure is reached in the system. When driving, keep an eye on the gauge(s) to measure air pressure buildup and become familiar with the common air pressure thresholds for your system. That way, if a problem is developing, you may be able to catch it before a serious loss of braking power occurs. (And you’ve all heard the air “bleed off” sound while passing a big rig. This air blast results when the air dryer or moisture ejectors expel water from the brake system. Get used to it if your coach has air brakes, as you’ll be hearing it quite a bit while driving.)
Regular testing of the integrity of an air brake system is important. To test your system, while the coach is parked, fully apply the brakes and hold them for at least two minutes while watching the air pressure gauge. Make sure the engine is off, the ignition key is in the “on” position, and the parking brake is off. Leakage, if any, should be less than 5 psi for the two minutes. Next, continue testing by applying the parking brake and pumping the brake pedal to release air. You should receive a visual and buzzer warning when the air pressure gauge drops below 60 psi.
Air Brake Problems
Contamination. Water contamination is the number one cause of air brake system problems. Water is pulled in with the air and must be removed from the system. An air dryer with moisture ejector is a common part of an air brake system. Air tanks have manual or automatic moisture ejector valves. On manual valve units, air must be ejected from the two or more air tanks on a daily basis.
Air leakage. If your compressor cycles frequently, it usually means that air is being lost somewhere in the air brake system. Although the frequent cycling of the compressor may keep system pressure within the acceptable margins, it is unsafe to operate a vehicle with a noticeable air brake system leak. Have the system checked by a professional immediately.
Air pressure gauges. Make it a habit to continually monitor the air pressure gauges for early signs of system malfunction, such as excessive or imbalanced air consumption. Leaks in the air system will affect braking ability.
Low pressure warning. By monitoring the air pressure gauges, you should be able to notice air leaks before getting a low pressure warning. If you do receive a low pressure warning, do not continue to operate the vehicle. Slow down and pull over to a safe stop on the side of the road. Driving with a low pressure warning can result in the parking brakes being automatically activated and lead to a loss of vehicle control as well as extended mechanical damage.
Note: Most air brake systems have true emergency brakes. The emergency brake features a heavy spring that is held in position by the normal air pressure. Should a loss of air pressure occur, the spring forces the brake pads/shoes into a “locked up” position to stop the vehicle immediately. This unit is typically mounted in tandem with the service brake chamber. While it should be a last resort, the park brake could be used to stop a motorhome. The danger is that it is akin to an on-off switch. The driver cannot control how fast or how hard the emergency brake applies. Because of a device called a “quick release,” all the air holding the spring is dumped in a few seconds, and the spring exerts its full force to apply the brakes, which could cause a skid with a resulting loss of positive control.
One of the best things to happen to vehicle safety in the past 25 years is the invention of antilock braking systems (ABS). The purpose of antilock braking is to help the driver control the vehicle during emergency braking by not allowing the system to lock up and cause the vehicle to skid. In the old days, good drivers accomplished this by pumping the pedal rapidly, but even the best race car drivers can only manage about 2.5 pumps per second. The average computer-controlled ABS system can pump the brakes between four to six times per second, thereby eliminating wheel lock and shortening braking distances.
The ABS system works by sensing tire and wheel rotation. If a wheel sensor detects an abrupt stop to wheel rotation, it sends a signal to the brake system computer, which in turn activates the pumping action of the pads against the rotor. With ABS, the driver can maintain braking control and steering control at the same time. That way, if you are not able to stop, at least you can control the direction in which you are going. In a skid, the vehicle will go only in the direction it was traveling when the brakes locked up, no matter how much you turn the wheel.
The problem with ABS is that, although it has been around for a number of years, many people don’t really know how to use it. They continue to pump the pedal, do not push the pedal hard enough, or don’t push the pedal long enough for ABS to take control. When using an ABS-equipped vehicle, the important thing to remember in an emergency situation is to always apply maximum pedal effort. You literally want to try to push the pedal through the floorboard. Push it down firmly and hold it down until the vehicle comes to a stop. Practice doing this in an empty parking lot after a rainstorm with proper technique, and you’ll be amazed at the shortened stopping distances and your ability to steer while stopping.
Never pump the brakes on an ABS-equipped unit. Doing so will interfere with the ABS operation, often making it ineffective.
On vehicles equipped with ABS, a warning light comes on momentarily and cycles off when you start the vehicle to let you know the system is functioning. If the light stays on, however, it means the ABS system is not working and that you need to get the system checked by a professional.
Brake System Maintenance
It is best to refer to your owners manual or contact your chassis manufacturer for specific brake maintenance procedures. However, some common recommendations follow:
Inspect the entire brake system at least once a year and lubricate slides or pins on disc-type brakes. This includes many checks, such as pedal free play, linkage lubrication, lining wear, rotor and drum condition, fluid condition, all brake lines/hoses for condition and leaks, power boost system operation, ABS operation, and warning systems operation. A brake system inspection is recommended after extended storage. Lubrication of linkage and caliper slides/pins is critical to prevent brake drag. Depending on storage conditions, vehicles that have high storage time have far more brake issues than units in constant operation.
Check and lubricate brake linkage at every oil change.
Check brake fluid level at every fuel stop.
Sure, Safe Stops
This article is intended to give you a general working knowledge of brake systems and their potential problems. Now what you need to do is tailor the knowledge to your system. Pull out the owners and chassis manuals for your motorhome and find out exactly what brake system the manufacturer has installed and whether it has ABS. Find out what type of brake fluid your motorhome uses, what general maintenance the system needs, and then locate a good brake shop.
I recommend using a commercial brake shop that specializes in heavy-duty trucks, buses, and motorhomes. The manager at my local RV service center told me that his shop hadn’t seen a brake job in six months, and that they only do three or four brake services a year. Your local commercial brake repair facility, however, may see as many as six brake services in one day, so odds are they really will know your vehicle.
Whatever you decide, make it a point to check the user-serviceable items on your brake system regularly and get the shop to give your motorhome’s brakes the once-over every year so you can be sure of a safe stop every time.