CDL Air Brake - Commercial Driver's License Air Brakes Foundation Brakes and Parts Questions and Answers — Questions and Answers
Question 1: Which of the following are the three main types of foundation brakes used on vehicles with air brake systems?
- S-cam, hydraulic, and wedge brakes
- Disc, magnetic, and S-cam brakes
- S-cam, disc, and wedge brakes (Correct answer)
- Drum, disc, and electromagnetic brakes
Correct answer: S-cam, disc, and wedge brakes
The three common types of foundation brakes are S-cam, disc, and wedge brakes. S-cam brakes are the most common type of drum brake found on heavy commercial vehicles.
Question 2: During a pre-trip inspection of S-cam brakes, you pull on the slack adjuster and find it has more than one inch of free play. What does this most likely indicate?
- The air compressor is failing.
- The brake drum is cracked.
- The brakes are out of adjustment. (Correct answer)
- The air tanks need to be drained.
Correct answer: The brakes are out of adjustment.
If a slack adjuster on an S-cam brake system moves more than approximately one inch where the pushrod attaches, it is an indication that the brakes are likely out of adjustment and need to be serviced. This is a critical safety check, as out-of-adjustment brakes are a common violation found during roadside inspections.
Question 3: What is the primary function of the S-cam in a foundation brake system?
- To convert air pressure into mechanical force.
- To store compressed air for the brake system.
- To automatically adjust for brake lining wear.
- To force the brake shoes apart and against the brake drum. (Correct answer)
Correct answer: To force the brake shoes apart and against the brake drum.
When the brakes are applied, the slack adjuster rotates the camshaft. The 'S' shape at the end of the camshaft, the S-cam, pushes against rollers on the brake shoes, forcing them apart and into contact with the brake drum to create friction and slow the vehicle.
Question 4: A driver is operating a truck with automatic slack adjusters. What is the correct procedure for ensuring they are working properly?
- Manually adjust them with a wrench during every pre-trip inspection to ensure they are tight.
- Perform several full, firm brake applications and then check for excessive pushrod travel.
- Check that the pushrod travel does not exceed the limit when pulling on it by hand; if it does, it indicates a problem. (Correct answer)
- Loosen the adjustment nut slightly to prevent the brakes from dragging.
Correct answer: Check that the pushrod travel does not exceed the limit when pulling on it by hand; if it does, it indicates a problem.
For automatic slack adjusters, the driver's role is to inspect, not manually adjust. During a pre-trip inspection with the brakes released, the driver should pull firmly on the pushrod. If it moves more than about one inch, it indicates a potential problem with the adjuster or foundation brakes that needs attention from a mechanic. Manually adjusting an automatic slack adjuster can mask underlying issues.
Question 5: Which component in a drum brake assembly converts the linear motion of the brake chamber pushrod into rotary motion to apply the brakes?
- Brake drum
- Slack adjuster (Correct answer)
- Return spring
- Anchor pin
Correct answer: Slack adjuster
The slack adjuster is the mechanical lever that connects the brake chamber's pushrod to the camshaft. Its primary functions are to transform the linear force of the pushrod into the twisting or rotary motion of the S-cam and to provide a way to adjust for lining wear.
Question 6: Compared to traditional S-cam drum brakes, what is a primary advantage of air disc brakes?
- They are less expensive to manufacture and replace.
- They are less prone to brake fade because they dissipate heat more efficiently. (Correct answer)
- They require more frequent manual adjustments to maintain performance.
- They have a longer service life due to thicker friction material.
Correct answer: They are less prone to brake fade because they dissipate heat more efficiently.
Air disc brakes have an open design that allows them to dissipate heat more effectively than enclosed drum brakes. This makes them less susceptible to brake fade, which is a reduction in stopping power caused by overheating, especially during long downhill grades or heavy braking.
CDL Air Brake - Commercial Driver's License Air Brakes Foundation Brakes and Parts Questions and Answers
Which of the following are the three main types of foundation brakes used on vehicles with air brake systems?