T4 ASE Brakes Exam with complete
Graded A+ solutions
Psi loss rate for Straight Truck
2 psi/minute
Psi loss rate for combination vehicle
less than 3 psi/minute
Service Brake Pressure Drop Test
Apply service brake to 90 psi. Loss rate should be less than 3 psi/minute for straight vehicles and less
than 4 psi/minute for combination vehicles.
Improperly adjusted brake shoes
Cause
1) brake imbalance
2) increased stopping distance
3) excessive heat and wear
Instability and premature wear
a 4 psi difference in the air delivery between axles is enough to cause instability and premature wear.
Troubleshooting poor brake performance due to air
1) inspect air compressor
2) verify pressure build up
3) look for clogged air compressor filter
4) possible slipping belt
5) possible air leaks
6) possible defective governor
Pressure Governors
1) regulates air going to system
2) adjustable pressure
3) defective governor or out of adjustment can cause compressor to function improperly.
4) designed to regulate the compressors output.
5) may be attached to compressor or externally mounted.
Air Compressors
1) Power is transferred to the air compressor by either an internal gear or by an external belt. Belt is
most common.
2)air can be drawn directly from the atmosphere through a filter.
3) Pistons push the air through a discharge valve to the supply reservoir.
Supply Reservoir "Wet Tank"
, 1) tank closest to the compressor.
2) "pop off valve" located on this tank to keep from over pressuring the system.
Pop-off valve
Also known as the pressure relief valve, exhaust valve, adjustable pressure limiting (APL) valve, or
overflow valve; this valve is the point of exit of excessive pressure buildup before damaging air
components.
Manual and Automatic Drain Valves
1) installed in each reservoir/tank to help purge the system from moisture.
2)Used in high humid or wet climates.
3) manual drain valves should be drained on a daily basis
Air Dryer
1) used to keep condensation out of the system using a desiccant material to remove moisture.
2
Air Compressor troubleshooting
1) can develop air leaks and pass oil into the brake system which will cause poor braking and or
grabbing or dragging.
2) clogged air intake filter will cause slow pressure build up.
Compressor oil checks
1) engine oil should be checked for compressors that get lubrication from engine.
2) Oil sump compressors have their own oil reservoirs and should be checked daily.
Compressor Pressure Testing
1) with engine running at idle; air pressure should build from 85-100 psi within 45 seconds.
2) if the engine is running at full governed rpm; pressure should build within 25 seconds and the
compressor should maintain a constant pressure of 100-120 psi.
Troubleshooting air leaks
1) if no leaks are found check to see if dirty air filter is restricting the intake of the compressor.
Gear Driven Compressor
1) must be meshed with the timing gear on the engine before installation. A misaligned gear can
cause poor pressure buildup.
2) Compressor timing can be achieved by aligning the timing marks on the gear and crankcase.
Governor Diagnosing
Graded A+ solutions
Psi loss rate for Straight Truck
2 psi/minute
Psi loss rate for combination vehicle
less than 3 psi/minute
Service Brake Pressure Drop Test
Apply service brake to 90 psi. Loss rate should be less than 3 psi/minute for straight vehicles and less
than 4 psi/minute for combination vehicles.
Improperly adjusted brake shoes
Cause
1) brake imbalance
2) increased stopping distance
3) excessive heat and wear
Instability and premature wear
a 4 psi difference in the air delivery between axles is enough to cause instability and premature wear.
Troubleshooting poor brake performance due to air
1) inspect air compressor
2) verify pressure build up
3) look for clogged air compressor filter
4) possible slipping belt
5) possible air leaks
6) possible defective governor
Pressure Governors
1) regulates air going to system
2) adjustable pressure
3) defective governor or out of adjustment can cause compressor to function improperly.
4) designed to regulate the compressors output.
5) may be attached to compressor or externally mounted.
Air Compressors
1) Power is transferred to the air compressor by either an internal gear or by an external belt. Belt is
most common.
2)air can be drawn directly from the atmosphere through a filter.
3) Pistons push the air through a discharge valve to the supply reservoir.
Supply Reservoir "Wet Tank"
, 1) tank closest to the compressor.
2) "pop off valve" located on this tank to keep from over pressuring the system.
Pop-off valve
Also known as the pressure relief valve, exhaust valve, adjustable pressure limiting (APL) valve, or
overflow valve; this valve is the point of exit of excessive pressure buildup before damaging air
components.
Manual and Automatic Drain Valves
1) installed in each reservoir/tank to help purge the system from moisture.
2)Used in high humid or wet climates.
3) manual drain valves should be drained on a daily basis
Air Dryer
1) used to keep condensation out of the system using a desiccant material to remove moisture.
2
Air Compressor troubleshooting
1) can develop air leaks and pass oil into the brake system which will cause poor braking and or
grabbing or dragging.
2) clogged air intake filter will cause slow pressure build up.
Compressor oil checks
1) engine oil should be checked for compressors that get lubrication from engine.
2) Oil sump compressors have their own oil reservoirs and should be checked daily.
Compressor Pressure Testing
1) with engine running at idle; air pressure should build from 85-100 psi within 45 seconds.
2) if the engine is running at full governed rpm; pressure should build within 25 seconds and the
compressor should maintain a constant pressure of 100-120 psi.
Troubleshooting air leaks
1) if no leaks are found check to see if dirty air filter is restricting the intake of the compressor.
Gear Driven Compressor
1) must be meshed with the timing gear on the engine before installation. A misaligned gear can
cause poor pressure buildup.
2) Compressor timing can be achieved by aligning the timing marks on the gear and crankcase.
Governor Diagnosing
