ASE MEDIUM/HEAVY TRUCK CERTIFICATION
T4 ASE BRAKES (MEDIUM/HEAVY DUTY
BRAKES) EXAM 2026/27!!
A+
Complete Blueprint Coverage
A+ 5 100%
QUESTIONS VERIFIED EXAM DOMAINS COVERED RATIONALES INCLUDED
CATEGORIES
Air Supply System, Governor, and Valves
Foundation Brakes, Chambers, and Slack Adjusters
Parking, Spring Brake, and Anti-Compounding Systems
Hydraulic Brakes, Power Assist, and Wheel Ends
ABS, ATC, ESC, and Electronic Brake Controls
STUVIAACTUALEXAM
,SECTION: AIR SUPPLY SYSTEM, GOVERNOR, AND VALVES
Q1.
A technician is diagnosing a tractor that takes longer than the FMVSS 121 limit to build air pressure from 85 to 100
psi. The compressor is cutting in at 100 psi and cutting out at 120 psi. What is the most likely cause of the slow
build-up time?
A. Restricted air intake to the compressor or a worn compressor unable to meet required delivery volume
B. Governor cut-out pressure set too high causing continuous loading
C. A leaking service brake chamber diaphragm on one axle only
D. An open circuit in the low-air warning buzzer
Correct Answer: A
Rationale:
Slow pressure build-up is typically caused by restricted compressor inlet, worn compressor rings/valves, or excessive system
leakage. Governor settings in the normal range do not by themselves slow the rate of pressure rise.
Q2.
During a pre-trip inspection the low-air warning lamp and buzzer activate when system pressure falls to 55 psi.
The technician must determine whether this meets FMVSS 121 requirements. What is the correct activation range
for the low-air warning?
A. The warning must activate before pressure drops below 60 psi in either reservoir circuit
B. The warning is permitted to activate only after pressure reaches 40 psi
C. The warning must remain silent until pressure is below 30 psi
D. The warning is required only on the secondary circuit and may ignore the primary
Correct Answer: A
Rationale:
FMVSS 121 requires the low-air warning to activate before pressure in either service reservoir drops below 60 psi. Activation
at 55 psi is acceptable; later activation violates the standard.
Q3.
A truck’s air dryer is discharging a large amount of oil-contaminated water every time the compressor unloads. The
technician suspects the purge cycle is normal but the contamination source is upstream. Which component is the
most likely source of excessive oil in the discharge air?
A. Worn compressor piston rings or cylinder bore allowing oil to pass into the air stream
B. A stuck-open dryer heater that vaporizes residual moisture only
C. A leaking service relay valve diaphragm on the rear axle
D. An overfilled windshield washer reservoir connected to the air system
Correct Answer: A
Rationale:
Oil-contaminated discharge from the dryer almost always originates at the compressor when rings or bore wear allow
crankcase oil to enter the compressed air. Dryer heaters and relay valves do not introduce engine oil.
, Q4.
A technician measures governor cut-in at 90 psi and cut-out at 105 psi. The system builds pressure normally but
the compressor cycles more frequently than expected. What adjustment or repair is indicated?
A. Adjust or replace the governor so cut-out is approximately 120–135 psi and cut-in is 20–25 psi lower
B. Lower the cut-out pressure further to reduce compressor run time
C. Disable the air dryer purge valve to stop pressure drop during unloading
D. Increase compressor speed by changing pulley ratio without addressing the governor
Correct Answer: A
Rationale:
Typical governor settings are cut-out near 120–135 psi with cut-in 20–25 psi lower. A 105 psi cut-out is too low, causing short
cycling. The governor must be adjusted or replaced to restore correct pressure range.
Q5.
While testing the dual air system, the technician drains the primary reservoir and observes that the secondary
pressure remains stable and the tractor protection valve stays open. What does this result indicate?
A. The dual-circuit check valve (or equivalent protection) is functioning and isolating the secondary circuit
B. The primary and secondary circuits are cross-connected and both will lose pressure
C. The tractor protection valve has failed in the closed position
D. The spring brake inversion valve is stuck in the emergency position
Correct Answer: A
Rationale:
Proper dual-circuit isolation allows the secondary reservoir to retain pressure when the primary is drained. Cross-connection or
a failed isolation device would cause both sides to drop together.
Q6.
A technician finds that the safety (pop-off) valve on the wet tank opens at 110 psi during a build-up test. Governor
cut-out is set at 125 psi. What is the correct corrective action?
A. Replace the safety valve; it must open at approximately 150 psi and is not a substitute for proper governor
control
B. Lower the governor cut-out to 105 psi so the safety valve never opens
C. Remove the safety valve because it is redundant with the governor
D. Plug the safety valve outlet to prevent air loss
Correct Answer: A
Rationale:
The safety valve is a backup that should open near 150 psi. Opening at 110 psi indicates a defective valve that must be
replaced; the governor remains the primary pressure-control device.
T4 ASE BRAKES (MEDIUM/HEAVY DUTY BRAKES) EXAM 2026/27!!... STUVIAACTUALEXAM
T4 ASE BRAKES (MEDIUM/HEAVY DUTY
BRAKES) EXAM 2026/27!!
A+
Complete Blueprint Coverage
A+ 5 100%
QUESTIONS VERIFIED EXAM DOMAINS COVERED RATIONALES INCLUDED
CATEGORIES
Air Supply System, Governor, and Valves
Foundation Brakes, Chambers, and Slack Adjusters
Parking, Spring Brake, and Anti-Compounding Systems
Hydraulic Brakes, Power Assist, and Wheel Ends
ABS, ATC, ESC, and Electronic Brake Controls
STUVIAACTUALEXAM
,SECTION: AIR SUPPLY SYSTEM, GOVERNOR, AND VALVES
Q1.
A technician is diagnosing a tractor that takes longer than the FMVSS 121 limit to build air pressure from 85 to 100
psi. The compressor is cutting in at 100 psi and cutting out at 120 psi. What is the most likely cause of the slow
build-up time?
A. Restricted air intake to the compressor or a worn compressor unable to meet required delivery volume
B. Governor cut-out pressure set too high causing continuous loading
C. A leaking service brake chamber diaphragm on one axle only
D. An open circuit in the low-air warning buzzer
Correct Answer: A
Rationale:
Slow pressure build-up is typically caused by restricted compressor inlet, worn compressor rings/valves, or excessive system
leakage. Governor settings in the normal range do not by themselves slow the rate of pressure rise.
Q2.
During a pre-trip inspection the low-air warning lamp and buzzer activate when system pressure falls to 55 psi.
The technician must determine whether this meets FMVSS 121 requirements. What is the correct activation range
for the low-air warning?
A. The warning must activate before pressure drops below 60 psi in either reservoir circuit
B. The warning is permitted to activate only after pressure reaches 40 psi
C. The warning must remain silent until pressure is below 30 psi
D. The warning is required only on the secondary circuit and may ignore the primary
Correct Answer: A
Rationale:
FMVSS 121 requires the low-air warning to activate before pressure in either service reservoir drops below 60 psi. Activation
at 55 psi is acceptable; later activation violates the standard.
Q3.
A truck’s air dryer is discharging a large amount of oil-contaminated water every time the compressor unloads. The
technician suspects the purge cycle is normal but the contamination source is upstream. Which component is the
most likely source of excessive oil in the discharge air?
A. Worn compressor piston rings or cylinder bore allowing oil to pass into the air stream
B. A stuck-open dryer heater that vaporizes residual moisture only
C. A leaking service relay valve diaphragm on the rear axle
D. An overfilled windshield washer reservoir connected to the air system
Correct Answer: A
Rationale:
Oil-contaminated discharge from the dryer almost always originates at the compressor when rings or bore wear allow
crankcase oil to enter the compressed air. Dryer heaters and relay valves do not introduce engine oil.
, Q4.
A technician measures governor cut-in at 90 psi and cut-out at 105 psi. The system builds pressure normally but
the compressor cycles more frequently than expected. What adjustment or repair is indicated?
A. Adjust or replace the governor so cut-out is approximately 120–135 psi and cut-in is 20–25 psi lower
B. Lower the cut-out pressure further to reduce compressor run time
C. Disable the air dryer purge valve to stop pressure drop during unloading
D. Increase compressor speed by changing pulley ratio without addressing the governor
Correct Answer: A
Rationale:
Typical governor settings are cut-out near 120–135 psi with cut-in 20–25 psi lower. A 105 psi cut-out is too low, causing short
cycling. The governor must be adjusted or replaced to restore correct pressure range.
Q5.
While testing the dual air system, the technician drains the primary reservoir and observes that the secondary
pressure remains stable and the tractor protection valve stays open. What does this result indicate?
A. The dual-circuit check valve (or equivalent protection) is functioning and isolating the secondary circuit
B. The primary and secondary circuits are cross-connected and both will lose pressure
C. The tractor protection valve has failed in the closed position
D. The spring brake inversion valve is stuck in the emergency position
Correct Answer: A
Rationale:
Proper dual-circuit isolation allows the secondary reservoir to retain pressure when the primary is drained. Cross-connection or
a failed isolation device would cause both sides to drop together.
Q6.
A technician finds that the safety (pop-off) valve on the wet tank opens at 110 psi during a build-up test. Governor
cut-out is set at 125 psi. What is the correct corrective action?
A. Replace the safety valve; it must open at approximately 150 psi and is not a substitute for proper governor
control
B. Lower the governor cut-out to 105 psi so the safety valve never opens
C. Remove the safety valve because it is redundant with the governor
D. Plug the safety valve outlet to prevent air loss
Correct Answer: A
Rationale:
The safety valve is a backup that should open near 150 psi. Opening at 110 psi indicates a defective valve that must be
replaced; the governor remains the primary pressure-control device.
T4 ASE BRAKES (MEDIUM/HEAVY DUTY BRAKES) EXAM 2026/27!!... STUVIAACTUALEXAM
