In this blog post, we will answer the following question: Do motorhomes have air brakes? We will explain how an air brake system works and what are the differences between hydraulic brakes and air brakes.
Do motorhomes have air brakes?
Not all motorhomes have air brakes. Lightweight motorhomes, such as Class B or Class C RVs have hydraulic brakes, while heavier motorhomes like Class A RVs need more braking capacity thus have air brakes.
The air brake system is so simple and works the same way on almost all motorhomes. They are so named because air pressure is used to activate the brakes once you put your foot on the pedal.
As we already mentioned, they are very common in heavy motorhomes such as Class A RVs because a great braking force is required to be able to completely stop these units.
How does an air brake system work?
The air is stored in several pressure tanks and a compressor is responsible for pressurizing it in the engine. Most, if not all systems, operate at 685 Kpa, 99.5 PSI, or 6.85 BAR of pressure. Some even have a humidity dryer to avoid cooling during the winter season, which has a cartridge inside that must be changed once a year.
Returning to the compressor, it pressurizes the air and transfers it to a fan and into the tanks. Once in the tanks, the air passes to two different systems:
The RV’s air brake system is very simple and works the same way on almost all trucks.
- Emergency system: This system releases the parking brake and saves some reserve air to use in the event of a leak. In most trucks, the parking brake system has springs in the air chambers that are in the axles so that the brakes work without problems.
To apply the parking brake, the control valves are actuated to the ON position, releasing air from the parking brake circuit. Therefore, the parking brake spring expands to move the stem and operates the S. This ensures that the brake shoes support the rear brake drums.
- Service brake system: This second system stops the vehicle when the brake pedal is pressed. In doing so, the air passes through the pedal valve and is modulated into the brake chambers.
The brake chambers in turn push the regulators, components that can be manual or automatic and that keep the brakes regulated. The manuals work exactly as the name implies: they must be adjusted manually to keep the brakes on.
The brake adjusters rotate a shaft that also turns the S-eccentric brake (named for its “S” shaped figure), pushing the brake shoes onto the drums. Inside the S eccentric shafts are the bearings, which must be checked every time the shoes are changed. These also need good lubrication.
Finally, there are air disc brakes, but they are not very common. Most brake shoes have a built-in indicator at the end of the pads that tells you when they need to be replaced. The replacement also includes accessories such as springs and bearings for the brakes.
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The differences between air brakes and hydraulic brakes
In a hydraulic system, the fluid is stored in a reservoir commonly known as a master cylinder. When you step on the brake pedal, fluid is pumped through the brake hoses or lines to the pistons mounted on each wheel.
These brake pistons push against two brake shoes, which expand and cause friction within a brake drum, or against a brake pad, which is clamped in a brake rotor. Below are the components of a hydraulic disc brake system:
- Brake reservoir: Contains hydraulic brake fluid
- Master cylinder: A device that pumps fluid from the reservoir to the brake lines that circulate throughout the vehicle.
- Brake lines: Braided rubber or steel hoses that run from the master cylinder to each brake calliper.
- Brake calliper: A steel housing that mounts to a fixed point on the brake rotor that contains a piston and brake pads.
- Brake Piston: A round rod that extends and pushes against a brake pad when hydraulic fluid is supplied from the master cylinder.
- Brake pads: A metal backing plate with a semi-metallic coating that grips the steel rotor.
- Brake rotor: A steel disc mounted on each wheel and hub that the pads grip to prevent the wheels from spinning.
The following components are unique to a foundation air brake system on an RV or bus:
- Air compressor: Pumps air into storage tanks for use in the brake system.
- Air Compressor Regulator: Controls the inlet and outlet point of the air compressor to maintain a fixed amount of air in the tank or tanks.
- Air tanks: Maintain compressed or pressurized air that will use the brake system.
- Drain Valves: Release valves on air tanks used to drain air when the vehicle is not in use.
- Foot valve (brake pedal): When pressed, the air is released from the reservoir tanks.
- Brake Chambers: Cylindrical container that houses a slack adjuster that moves a diaphragm or cam mechanism.
- Push Rod: A piston-like steel rod that connects the brake chamber to the slack adjuster. When pressed, the brakes are released. If it is extended, the brakes are on.
- Slack Adjusters: An arm connects the push rod with the brake s-cam to adjust the distance between the brake shoes.
- S-cam brake: An s-shaped cam that pushes the brake shoes out and against the brake drum.
- Brake shoe: Steel mechanism with a coating that causes friction against the brake drum
How to release air brakes?
RV air brakes work in the opposite way as hydraulic brakes. When the brake is pressed to a hydraulic system, it pushes the callipers onto the rotor, causing the truck to stop.
Air brakes work the exact opposite. The air in the brake system actually pushes the callipers away from the drums. When the brake pedal is depressed, this releases air from the brake lines and applies pressure to the drum. This is why it is essential to have an air supply to the brake lines.
- Start the RV by turning the ignition key to the “Start” position. Some RVs have a push-button start and will require turning the key to the “On” position and then pressing the button to start the truck.
- Locate the air pressure gauge on the dash. This gauge will be a small gauge labelled “air pressure.”
- Watch the needle on the air pressure gauge rise while the truck is idling. An indicator will make a buzz as well.
- Allow air pressure to build up to 120 pounds of air. The buzz will stop at 60 kilos. You will hear the airbags settle under the truck once the pressure stops building.
- Press the yellow button labelled “Parking” while your foot is on the brake pedal. This will release the truck air brakes.
- Press the red button labelled “trailer” if a trailer is attached. This will release the trailer brakes. Keep your foot on the brake pedal to prevent the truck from moving.
Air brakes (also known as spring brakes) have been around almost as long as the compressor and the automobile. These heavy but reliable systems are used where safety takes precedence over overweight, and are responsible for keeping more RVs out of more ditches than any development since the drum brake.
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FAQ on Do motorhomes have air brakes?
How does the air brake system work?
The pneumatic brake or air brake is a type of brake whose actuation is carried out by compressed air. It uses pistons that are fed with compressed air tanks by means of a compressor, whose control is carried out by valves.
What are the parts of air brakes?
The parts of the air brakes are:
- Air compressor.
- Air compressor governor.
- Air storage tanks.
- Air tank drains.
- Alcohol evaporator
- Safety valve.
What is a dual air brake system?
A dual air brake system has two separate air brake systems that use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system generally operates the normal brakes on the axle or rear axles.