DTC Brake Systems 5 — Questions and Answers
Question 1: Under the CVSA out-of-service criteria, what share of a vehicle's service brakes found defective or out of adjustment puts the vehicle out of service?
- 20% or more (Correct answer)
- 10% or more
- 50% or more
- 30% or more
Correct answer: 20% or more
Under the CVSA 20 percent criterion, a vehicle goes out of service when defective brakes reach 20% or more of its service brakes.
Question 2: What does a pressure protection valve do in an air brake system?
- It regulates brake chamber application pressure
- It limits compressor discharge temperature
- It quickly exhausts the trailer emergency line
- It keeps air in the brake reservoirs by closing an accessory circuit when pressure falls below its set point (Correct answer)
Correct answer: It keeps air in the brake reservoirs by closing an accessory circuit when pressure falls below its set point
Pressure protection valves close off non-brake accessories so a leak there cannot drain air the brakes need.
Question 3: An air disc brake has uneven inboard/outboard pad wear and the caliper fails to release fully. What is the most likely cause?
- Governor cut-out is set too low
- Seized or binding caliper guide pins (Correct answer)
- Wheel speed sensor is open
- Desiccant cartridge is too large
Correct answer: Seized or binding caliper guide pins
Binding guide pins stop the caliper from floating, which causes uneven pad wear and drag.
Question 4: A trailer built after March 1, 1998 (through 2009) has its ABS warning lamp lit after power-up. Where is that external lamp usually mounted?
- On the kingpin plate
- On the front left corner of the trailer
- On the left rear side of the trailer (Correct answer)
- Inside the trailer near the landing gear
Correct answer: On the left rear side of the trailer
Federal rules called for an external trailer ABS lamp on the left side near the rear so the driver can see it in the mirror.
Question 5: What does the governor do when system pressure reaches cut-out pressure?
- It purges the air dryer through the brake chambers
- It sends air to the compressor unloader mechanism so the compressor stops pumping (Correct answer)
- It opens the reservoir safety valve
- It shuts off fuel to the compressor
Correct answer: It sends air to the compressor unloader mechanism so the compressor stops pumping
At cut-out the governor pressurizes the unloader, which puts the compressor in its unloaded cycle until cut-in.
Question 6: In a typical hydraulic drum brake system, what is the main job of the residual check valve in the master cylinder outlet?
- To keep a slight pressure in the lines so wheel cylinder cups stay sealed and air stays out (Correct answer)
- To delay front disc application
- To warn the driver about low fluid level
- To balance front-to-rear braking under heavy load
Correct answer: To keep a slight pressure in the lines so wheel cylinder cups stay sealed and air stays out
A small residual pressure keeps the wheel cylinder cup lips seated and helps keep air out of the system.
Question 7: After a full service brake application, applied air leakage on a combination vehicle should not go above what rate?
- 2 psi per minute
- 3 psi per minute
- 4 psi per minute (Correct answer)
- 8 psi per minute
Correct answer: 4 psi per minute
Applied leakage limits are 3 psi per minute for a single vehicle and 4 psi per minute for a combination vehicle.
DTC Brake Systems 5
Under the CVSA out-of-service criteria, what share of a vehicle's service brakes found defective or out of adjustment puts the vehicle out of service?