Michigan CDL: Air Brakes Practice Test

Not quite. The answer is 150 psi.

The safety valve, which protects the system from excessive pressure, is usually set to open when the system reaches 150 psi.

Not quite. The answer is Evacuate the air from the brake chambers.

In today's CMVs, you apply or release the parking brakes with the parking brake control on the dashboard. This is a diamond-shaped, yellow, push-pull control knob. To apply the parking brakes, pull out the knob to evacuate the air from each brake chamber, which will cause the spring brakes to come on.

Not quite. The answer is 3,000 pounds.

The most common type of air brake chamber in trucks and trailers is the Type 30. The type number gives the area of the diaphragm. Thus, the diaphragm in a Type 30 brake chamber has an area of 30 square inches. If a force of 100 psi (pounds per square inch) is applied to the diaphragm, the resulting pushrod force will equal 100 × 30 = 3,000 pounds.

Not quite. The answer is always keep it on the "normal" setting.

Some older vehicles have a manually controlled front brake limiting valve to reduce air pressure to the front brakes when traveling on slippery roads. However, tests have since shown that front wheel braking is unlikely to cause a skid even on slippery roads. And reducing the air pressure reduces the vehicle's stopping power. Keep the front brake limiting valve on the "normal" setting at all times.

Not quite. The answer is 150 psi

The safety valve, which protects the system from excessive pressure, is usually set to open when the system reaches 150 psi.

Not quite. The answer is To show how much pressure is in the air tanks.

The air supply pressure gauge is connected to the air tank (or possibly to both tanks in a dual air tank system). It shows how much air pressure is in the air tanks.

Not quite. The answer is Stop and park safely.

If the low air pressure warning comes on, there may be a leak in the system. Pull over and park your vehicle as soon as possible. If the air pressure drops too low, the spring brakes will come on automatically, which will stop your vehicle – but not necessarily in a safe way. Sudden application of the spring brakes while you're driving might result in a skid.

Not quite. The answer is spring brakes.

In some vehicles, a control handle on the dashboard 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.

Not quite. The answer is 100 psi.

In a typical air brake system, pumping by the air compressor should start at about 100 psi and stop at about 125 psi. (Check the manufacturer's specifications for the actual figures.)

Not quite. The answer is forces the brake shoes against the inside of the brake drum.

When the S-cam rotates, it forces the brake shoes apart and against the inside of the brake drum. Friction will slow the rotation of the wheel.

Not quite. The answer is All of the above

Brake fade and failure are usually due to incorrect use of the brakes that causes excess heat. This may be caused by incorrect brake adjustment, not enough reliance on engine braking, excessive use of the service brakes, or another mechanical factor or human error.

Not quite. The answer is 1/4 inch

According to federal regulations on air brakes, the brake linings of non-steering-axle brakes must be at least 1/8 inch thick for disc brakes or at least 1/4 inch thick for drum brakes. The brake linings of steering-axle drum brakes can be thinner.

Not quite. The answer is (b) or (c)

Before measuring pushrod stroke, chock the wheels of your vehicle. For an accurate measurement, make sure that tank air pressures are 90–100 psi. Here are two different methods for measuring pushrod stroke: (1) Make a mark on the pushrod where it exits the brake chamber, and see how far it moves from the brake chamber when the service brakes are applied. (2) Measure the length of the part of the pushrod from the brake chamber to the center of the clevis pin, and see how much it increases when the brakes are applied. To perform either method efficiently, two people may be needed: one to apply the service brakes, and one to make the appropriate measurements before and after the brakes are applied. There are also various pushrod stroke measuring tools available commercially that you can use.

Not quite. The answer is vehicles with air brakes.

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. Another type of warning is the “wig wag.” This device drops a mechanical arm into your view when the pressure in the system drops between the dangerous levels.

Not quite. The answer is It depends on the adjustment of the service brakes.

The braking power of spring brakes depends on the brakes being in adjustment. If the brakes are not adjusted properly, neither the service brakes nor the emergency or parking brakes will work right.

Not quite. The answer is The vehicle was manufactured after 1998.

Vehicles manufactured after 1998 are required to have anti-lock brakes. They also have a yellow ABS malfunction lamp.

Not quite. The answer is 2 psi per minute.

To test the static air leakage rate, turn off the engine, release the parking brake (i.e., push the parking brake control knob in), and see how fast the air pressure drops. The air pressure should drop less than 2 psi in one minute for a single vehicle and less than 3 psi in one minute for a combination vehicle.

Not quite. The answer is you will still have enough air in the storage tanks to apply the air brakes several times.

The air storage tanks hold enough air for you to apply the air brakes several times.

Not quite. The answer is Clockwise until the bolt stops turning

Adjusting an automatic slack adjuster is an emergency procedure that you should perform only if your brakes have failed on the road. Most automatic slack adjusters have a hexagonal (six-sided) adjusting bolt. For this type of slack adjuster, first turn the bolt clockwise until it stops turning. If the slack adjuster has a square adjusting bolt, first turn the bolt counterclockwise until it stops turning. Then with either type of bolt, back it off about half a turn in the opposite direction.

Not quite. The answer is the service brake valve and the parking brake valve.

A spring parking brake can exert a force of up to 2,000 pounds, and a Type 30 service brake chamber can exert a force of 3,000 pounds or more. If both forces were applied at the same time (which is called compounding the service and parking brakes), the combined force could damage parts of the braking system. To prevent this, most modern commercial vehicles employ an anti-compounding valve. One input port of this valve is connected to the service brake valve (foot valve), and another input port is connected to the parking brake valve (yellow knob). Suppose you set the parking brakes by pulling out the yellow knob. That valve will send air to the anti-compounding valve. If you also press the brake pedal, the foot valve will also send air to the anti-compounding valve. Then the anti-compounding valve will partially release the parking brakes by the same amount of force that the foot valve has applied.