Cdl air brake manual

The water can freeze in cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank. Be sure that you drain the air tanks completely. Each air tank is equipped with a drain valve in the bottom. There are 2 types:.

Automatic air tanks are available with electric heating devices. These help prevent freezing of the automatic drain in cold weather. Some air brake systems have an alcohol evaporator to put alcohol into the air system.

This helps to reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside the system can make the brakes stop working. Check the alcohol container and fill up as necessary. Daily air tank drainage is still needed to get rid of water and oil unless the system has automatic drain valves. A safety relief valve is installed in the first tank the air compressor pumps air to. The safety valve protects the tank and the rest of the system from too much pressure.

The valve is usually set to open at psi. If the safety valve releases air, something is wrong. Have the fault fixed by a mechanic. You engage the brakes by pushing down the brake pedal It is also called a foot valve or treadle valve.

Pushing the pedal down harder applies more air pressure. Letting up on the brake pedal reduces the air pressure and releases the brakes. Releasing the brakes lets some compressed air go out of the system, so the air pressure in the tanks is reduced. It must be made up by the air compressor. Pressing and releasing the pedal unnecessarily can let air out faster than the compressor can replace it.

If the pressure gets too low, the brakes will not work. Foundation brakes are used at each wheel. The most common type is the S-cam drum brake. The parts of the brake are discussed below. Brake Drums, Shoes, and Linings. The wheels are bolted to the drums. The braking mechanism is inside the drum.

To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle and creates heat. The heat a drum can take without damage depends on how hard and how long the brakes are used. Too much heat can make the brakes stop working. S-cam Brakes. When you push the brake pedal, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft.

The S-cam forces the brake shoes away from one another and presses them against the inside of the brake drum. When you release the brake pedal, the S-cam rotates back and a spring pulls the brake shoes away from the drum, letting the wheels roll freely again. See Figure 5. The CamLaster brake has 2 key design differences over traditional S-cam brakes. One feature is a completely internal adjustment system which is designed to continually keep the brake in proper adjustment.

S-cam brakes, on the other hand, require an external slack adjuster. The second feature is a unique cam design that applies the brake shoe. Unlike a standard drum brake that has either a single or double anchor-pin brake, the CamLaster slides the shoes down an inclined ramp on a cam to evenly contact the brake drum. Wedge Brakes. In this type of brake, the brake chamber push rod pushes a wedge directly between the ends of 2 brake shoes. This shoves them apart and against the inside of the brake drum.

Wedge brakes may have a single brake chamber or 2 brake chambers that push wedges in at both ends of the brake shoes. Wedge type brakes may be self-adjusting or may require manual adjustment.

Disc Brakes. In air-operated disc brakes, air pressure acts on a brake chamber and slack adjuster, like S-cam brakes. The pressure of the brake chamber on the slack adjuster turns the power screw. The power screw clamps the disc or rotor between the brake lining pads of a caliper, similar to a large c-clamp.

All vehicles with air brakes have a pressure gauge connected to the air tank. If the vehicle has a dual air brake system, there will be a gauge for each half of the system or a single gauge with two needles. Dual systems will be discussed later.

These gauges tell you how much pressure is in the air tanks. This gauge shows how much air pressure you are applying to the brakes. This gauge is not on all vehicles. Increasing application pressure to hold the same speed means the brakes are fading. You should slow down and use a lower gear. Brakes that are of adjustment, air leaks, or mechanical problems can also cause the need for increased pressure. A low air pressure warning signal is required on vehicles with air brakes. The warning is usually a red light.

A buzzer may also come on. An automatic wig wag will rise out of your view when the pressure in the system goes above 55 and 75 psi. It will not stay in place until the pressure in the system is above 55 psi. Drivers behind you must be warned when you put your brakes on. The air brake system does this with an electric switch that works by air pressure.

The switch turns on the brake lights when you put on the air brakes. Some vehicles made before have a front brake limiting valve and a control in the cab.

Limiting valves were used to reduce the chance of the front wheels skidding on slippery surfaces. However, they actually reduce the stopping power of the vehicle. Front wheel braking is good under all conditions.

Tests have shown front wheel skids from braking are not likely even on ice. Many vehicles have automatic front wheel limiting valves. They reduce the air to the front brakes except when the brakes are put on very hard 60 psi or more application pressure. The driver cannot control these valves. All trucks, truck tractors, and buses must be equipped with emergency brakes and parking brakes. They must be held on by mechanical force because air pressure can eventually leak away.

Spring brakes are usually used to meet these needs. Powerful springs are held back by air pressure when driving.

If the air pressure is removed, the springs put on the brakes. A parking brake control in the cab allows the driver to let the air out of the spring brakes. This lets the springs put the brakes on. A leak in the air brake system, which causes all the air to be lost, will also cause the springs to put on the brakes. Tractor and straight truck spring brakes will come fully on when air pressure drops to a range of 20 to 45 psi typically 20 to 30 psi.

Do not wait for the brakes to come on automatically. When the low air pressure warning light, and buzzer first come on, bring the vehicle to a safe stop right away, while you can still control the brakes. The braking power of spring brakes depends on the brakes being in adjustment. In newer vehicles with air brakes, you put on the parking brakes using a diamond-shaped, yellow, push-pull control knob. You pull the knob out to put the parking brakes spring brakes on, and push it in to release them.

On older vehicles, the parking brakes may be controlled by a lever. Use the parking brakes whenever you park. Never push the brake pedal down when the spring brakes are on.

If you do, the brakes could be damaged by the combined forces of the springs and the air pressure. Many brake systems are designed so this will not happen. Not all systems are set up that way, and those that are may not always work.

It is much better to develop the habit of not pushing the brake pedal down when the spring brakes are on. Modulating Control Valves. In some vehicles a control handle on the dash board may be used to apply the spring brakes gradually. This is called a modulating valve. It is spring-loaded so you have a feel for the braking action. The more you move the control lever, the harder the spring brakes come on.

They work this way so you can control the spring brakes if the service brakes fail. When parking a vehicle with a modulating control valve, move the lever as far as it will go and hold it in place with the locking device. Dual Parking Control Valves. When main air pressure is lost, the spring brakes come on. Some vehicles, such as buses, have a separate air tank which can be used to release the spring brakes.

This is so you can move the vehicle in an emergency. One of the valves is a push-pull type and is used to put on the spring brakes for parking. When you push the control in, air from the separate air tank releases the spring brakes so you can move.

When you release the button, the spring brakes come on again. There is only enough air in the separate tank to do this a few times.

Therefore, plan carefully when moving. Otherwise, you may be stopped in a dangerous location when the separate air supply runs out.

Truck tractors with air brakes built on or after March 1, , and other air brakes vehicles trucks, buses, trailers, and converter dollies built on or after March 1, , are required to be equipped with anti-lock brakes. Many commercial vehicles built before these dates have been voluntarily equipped with ABS. Check the certification label for the date of manufacture to determine if your vehicle is equipped with ABS.

ABS is a computerized system that keeps your wheels from locking up during hard brake applications. On newer vehicles, the malfunction lamp comes on at start-up for a bulb check, and then goes out quickly. On older systems, the lamp could stay on until you are driving over 5 mph. Most heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has 2 separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, etc.

One system typically operates the regular brakes on the rear axle or axles. The other system operates the regular brakes on the front axle and possibly one rear axle. Both systems supply air to the trailer if there is one. Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of psi pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges or needles, if the system has 2 needles in one gauge.

Pay attention to the low air pressure warning light and buzzer. The warning light and buzzer should shut off when air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 55 psi. The warning light and buzzer should come on before the air pressure drops below 55 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle.

If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. This means it will take you longer to stop. Bring the vehicle to a safe stop, and have the air brakes system fixed. This device allows air to flow in one direction only. The check valve keeps air from going out if the air compressor develops a leak. You should use the basic 7-step inspection procedure described in Section 2 to inspect your vehicle.

There is more to inspect on a vehicle with air brakes than one without them. These components are discussed below, in the order that they fit into the 7-step method. Check the air compressor drive belt if the compressor is belt-driven.

If the air compressor is belt-driven, check the condition and tightness of the belt. It should be in good condition.

Check slack adjusters on S-cam brakes. Park on level ground and chock the wheels to prevent the vehicle from moving. Release the parking brakes so you can move the slack adjusters. Use gloves and pull hard on each slack adjuster that you can reach. If a slack adjuster moves more than about one inch where the push rod attaches to it, it probably needs adjustment.

Adjust it or have it adjusted. Vehicles with too much brake slack can be very hard to stop. Out-of-adjustment brakes are the most common problem found in roadside inspections. Be safe. Check the slack adjusters. All vehicles built since have automatic slack adjusters. Even though automatic slack adjusters adjust themselves during full brake applications, they must be checked. Automatic adjusters should not have to be manually adjusted except when performing maintenance on the brakes and during installation of the slack adjusters.

In a vehicle equipped with automatic adjusters, when the pushrod stroke exceeds the legal brake adjustment limit, it is an indication that a mechanical problem exists in the adjuster itself, a problem exists with the related foundation brake components, or the adjuster was improperly installed.

The manual adjustment of an automatic adjuster to bring a brake pushrod stroke within legal limits is generally masking a mechanical problem and is not fixing it. Interactive online CDL Quizzes.

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