Colorado CDL Air Brakes cheat sheet

Three-part system: compressor with governor and reservoirs supplies air; foot valve and lines deliver it; brake chambers, slack adjusters, and foundation brakes apply it.

The compressor is engine-driven and governed: it pumps air until about 100-125 psi, then the governor unloads it; pumping resumes around 85 psi (cut-in) so pressure stays in a safe band.

A safety valve on the first (supply) reservoir protects the system; it is normally set near 150 psi and vents if the governor fails and pressure climbs too high.

The supply reservoir feeds the primary (rear brake) and secondary (front brake) circuits through one-way check valves, so a single circuit failure cannot drain the whole system.

The foot valve (treadle) is a graduated control: light pressure gives light braking, harder pressure gives harder braking - it is not an on/off switch.

A dual-circuit system keeps one circuit working if the other fails; expect a low-pressure warning and reduced braking, so stop and check the system.

The alcohol evaporator (or air dryer) removes moisture; water in the system can freeze in cold weather and cause brake failure.

Spring brakes apply by spring force when air pressure is released; they release only when enough air pressure builds to compress the spring.

The parking (spring) brake control is a yellow diamond knob: pull to apply, push to release; a tractor protection valve and trailer lines complete the combination-vehicle circuit.

Static leak test: with a fully charged system, engine off, and brakes released, watch the gauge for one minute - a loss of more than 2 psi in one minute signals a leak.

Applied leak test: hold the foot valve fully down for one minute with the system charged - more than 3 psi loss in one minute is excessive.

Low-pressure warning test: with the engine off, pump the foot valve until the warning light and buzzer come on; they must activate before pressure drops below 60 psi.

Spring brake test: with the system charged, pull the yellow knob and then fan the foot valve to bleed air; the spring brakes must set (vehicle cannot be moved) by the time pressure falls to about 45 psi.

Compressor build-up check: with the engine at operating rpm and the system drained, pressure should rise from 85 to 100 psi in about 45 seconds (a common acceptable range is 30-60 seconds).

Governor cut-out check: the compressor should stop pumping at the governed cut-out pressure shown on the gauge, typically near 125 psi; continued rise points to a governor or safety-valve fault.

Before each trip, drain moisture from the reservoirs (usually daily) and check that the air dryer is working; water and oil carryover damage valves and can freeze.

Walk-around check: look for cracked or damaged hoses, loose clamps, chafed lines, and audible leaks; a hiss at a chamber or valve means the vehicle is out of service until repaired.

Test the slack adjusters by pulling them by hand - they should not move more than about 1 inch at the push rod; more free play means adjustment is needed.

Manual slack adjusters can be adjusted, but automatic slack adjusters must not be manually adjusted to correct a chronic problem - find the cause instead.

Push-rod stroke is the key adjustment measure: too long a stroke means the chamber uses up its travel before full braking force reaches the drum.

A common rule of thumb: with the brakes fully applied, the push rod should not travel more than about 1 inch for each 10 psi of application pressure (check the manufacturer's limit).

Warning devices include a low-pressure light and buzzer (below about 60 psi) and a parking brake indicator; both must be checked before driving.

A rapid loss of air pressure while driving, or pressure that will not build, means stop safely and do not continue - the brakes may fail completely.

If the low-pressure warning sounds while driving, move to a safe stop, apply the parking brake, and check the system; do not keep driving to the destination.

A dragging brake (hot drum, smoke, or a pull to one side) can be caused by a stuck slack adjuster, a broken spring, or a seized S-cam; it overheats and fades.

Brake fade from overheated drums reduces stopping power; use a lower gear and engine braking on long downgrades rather than riding the brakes.

A compressor that never reaches cut-out pressure, or a gauge that keeps climbing, indicates a governor or safety-valve fault - get it checked before the trip.

The parking brake is the spring brake: pulling the yellow knob releases air and lets the spring apply the brakes; it stays applied without air pressure.

Never use the parking brake for a normal stop while moving at speed, except in an emergency; use the foot valve for routine braking.

On a long downgrade, select a low gear before starting down and use engine braking; the foot brake is for control, not for holding speed continuously.

If the service brakes fail, apply the parking (spring) brake gradually, downshift, and look for a runaway ramp or safe escape route.

Stopping distance grows with speed and weight: at highway speed an air-braked combination vehicle needs far more distance than a car, so increase following distance.

Before backing or parking, set the parking brake and chock the wheels when parked on a grade; a spring brake can be released accidentally if air is restored.

In cold weather, drain moisture and allow air pressure to build fully before moving; frozen lines or valves can prevent the brakes from releasing or applying.

After a hard stop or long downgrade, let the brakes cool before parking; setting the parking brake on hot drums can cause cracking or fade on the next use.

When the low-air warning activates, stop as soon as it is safe, set the parking brake, and do not move the vehicle until the system is repaired.

The compressor is engine-driven and supplies air to the reservoirs; it does not store air itself.

The governor controls compressor cut-in and cut-out pressure, typically unloading near 125 psi and resuming near 85 psi.

The safety valve on the supply reservoir vents at about 150 psi if the governor fails and pressure keeps rising.

The foot valve (treadle) is a graduated valve: more pedal travel admits more air and produces more braking force.

The supply reservoir feeds the primary and secondary circuits through one-way check valves, so a failed circuit cannot drain the other.

The alcohol evaporator or air dryer removes moisture from the system to prevent freeze-ups and corrosion.

Spring brakes apply by spring force when air is released and release only when air pressure is restored.

The yellow diamond parking brake knob controls the spring brakes: pull to apply, push to release.

Static leak test: fully charged system, engine off, brakes released - more than 2 psi loss in one minute is a leak.

Applied leak test: hold the foot valve down for one minute - more than 3 psi loss in one minute is excessive.

Low-pressure warning check: with the engine off, pump the foot valve until the light and buzzer activate; they must come on before pressure falls below 60 psi.

Spring brake check: with the system charged, pull the yellow knob and fan the foot valve; the spring brakes must set by about 45 psi.

Compressor build-up: from 85 to 100 psi should take roughly 45 seconds at operating rpm (about 30-60 seconds is a common acceptable range).

Governor cut-out: the compressor should stop pumping near the governed pressure, typically about 125 psi.

Drain moisture from the reservoirs daily and check that the air dryer is working.

Walk-around inspection: look for cracked hoses, loose clamps, chafed lines, and audible leaks.

Slack adjuster check: pull each slack adjuster by hand; more than about 1 inch of free play at the push rod means adjustment is needed.

Push-rod stroke measures adjustment: too long a stroke means the chamber runs out of travel before full braking force reaches the drum.

Rule of thumb: with the brakes fully applied, push-rod travel should not exceed about 1 inch per 10 psi of application pressure (check the manufacturer's limit).

Manual slack adjusters can be adjusted, but automatic slack adjusters should not be manually adjusted to hide a chronic problem.

Warning devices: a low-pressure light and buzzer (below about 60 psi) and a parking brake indicator must be checked before driving.

A rapid loss of air pressure while driving, or pressure that will not build, means stop safely - the brakes may fail completely.

A dragging brake (hot drum, smoke, pull to one side) can come from a stuck slack adjuster, broken spring, or seized S-cam.

Brake fade from overheated drums reduces stopping power; use a lower gear and engine braking on long downgrades.

A compressor that never reaches cut-out, or a gauge that keeps climbing, points to a governor or safety-valve fault.

If the low-pressure warning sounds while driving, stop safely, set the parking brake, and check the system before continuing.

The parking brake is the spring brake: pulling the yellow knob releases air and lets the spring apply the brakes; it stays applied without air pressure.

Do not use the parking brake for a normal moving stop except in an emergency; use the foot valve for routine braking.

On a long downgrade, select a low gear before starting down and use engine braking; the foot brake is for control, not for holding speed continuously.

If the service brakes fail, apply the parking (spring) brake gradually, downshift, and look for a runaway ramp or safe escape route.

Stopping distance grows with speed and weight; an air-braked combination vehicle needs far more distance than a car, so increase following distance.

Before backing or parking, set the parking brake and chock the wheels on a grade; a spring brake can release accidentally if air is restored.

In cold weather, drain moisture and let pressure build fully before moving; frozen lines or valves can prevent release or application.

After a hard stop or long downgrade, let the brakes cool before parking; setting the parking brake on hot drums can cause cracking or fade.

When the low-air warning activates, stop as soon as it is safe, set the parking brake, and do not move until the system is repaired.

The compressor is engine-driven and supplies air to the reservoirs; it does not store air itself.

The governor controls compressor cut-in and cut-out pressure, typically unloading near 125 psi and resuming near 85 psi.

The safety valve on the supply reservoir vents at about 150 psi if the governor fails and pressure keeps rising.

The foot valve (treadle) is a graduated valve: more pedal travel admits more air and produces more braking force.

The supply reservoir feeds the primary and secondary circuits through one-way check valves, so a failed circuit cannot drain the other.

The alcohol evaporator or air dryer removes moisture from the system to prevent freeze-ups and corrosion.

Spring brakes apply by spring force when air is released and release only when air pressure is restored.

The yellow diamond parking brake knob controls the spring brakes: pull to apply, push to release.

Static leak test: fully charged system, engine off, brakes released - more than 2 psi loss in one minute is a leak.

Applied leak test: hold the foot valve down for one minute - more than 3 psi loss in one minute is excessive.

Low-pressure warning check: with the engine off, pump the foot valve until the light and buzzer activate; they must come on before pressure falls below 60 psi.

Spring brake check: with the system charged, pull the yellow knob and fan the foot valve; the spring brakes must set by about 45 psi.

Compressor build-up: from 85 to 100 psi should take roughly 45 seconds at operating rpm (about 30-60 seconds is a common acceptable range).

Governor cut-out: the compressor should stop pumping near the governed pressure, typically about 125 psi.

Drain moisture from the reservoirs daily and check that the air dryer is working.

Walk-around inspection: look for cracked hoses, loose clamps, chafed lines, and audible leaks.

Slack adjuster check: pull each slack adjuster by hand; more than about 1 inch of free play at the push rod means adjustment is needed.

Push-rod stroke measures adjustment: too long a stroke means the chamber runs out of travel before full braking force reaches the drum.

Rule of thumb: with the brakes fully applied, push-rod travel should not exceed about 1 inch per 10 psi of application pressure (check the manufacturer's limit).

Manual slack adjusters can be adjusted, but automatic slack adjusters should not be manually adjusted to hide a chronic problem.

Warning devices: a low-pressure light and buzzer (below about 60 psi) and a parking brake indicator must be checked before driving.

A rapid loss of air pressure while driving, or pressure that will not build, means stop safely - the brakes may fail completely.

A dragging brake (hot drum, smoke, pull to one side) can come from a stuck slack adjuster, broken spring, or seized S-cam.

Brake fade from overheated drums reduces stopping power; use a lower gear and engine braking on long downgrades.

A compressor that never reaches cut-out, or a gauge that keeps climbing, points to a governor or safety-valve fault.

If the low-pressure warning sounds while driving, stop safely, set the parking brake, and check the system before continuing.

The parking brake is the spring brake: pulling the yellow knob releases air and lets the spring apply the brakes; it stays applied without air pressure.

Do not use the parking brake for a normal moving stop except in an emergency; use the foot valve for routine braking.

On a long downgrade, select a low gear before starting down and use engine braking; the foot brake is for control, not for holding speed continuously.

If the service brakes fail, apply the parking (spring) brake gradually, downshift, and look for a runaway ramp or safe escape route.

Stopping distance grows with speed and weight; an air-braked combination vehicle needs far more distance than a car, so increase following distance.

Before backing or parking, set the parking brake and chock the wheels on a grade; a spring brake can release accidentally if air is restored.

In cold weather, drain moisture and let pressure build fully before moving; frozen lines or valves can prevent release or application.

After a hard stop or long downgrade, let the brakes cool before parking; setting the parking brake on hot drums can cause cracking or fade.

When the low-air warning activates, stop as soon as it is safe, set the parking brake, and do not move until the system is repaired.

The compressor is engine-driven and supplies air to the reservoirs; it does not store air itself.

The governor controls compressor cut-in and cut-out pressure, typically unloading near 125 psi and resuming near 85 psi.

The safety valve on the supply reservoir vents at about 150 psi if the governor fails and pressure keeps rising.

The foot valve (treadle) is a graduated valve: more pedal travel admits more air and produces more braking force.

The supply reservoir feeds the primary and secondary circuits through one-way check valves, so a failed circuit cannot drain the other.

The alcohol evaporator or air dryer removes moisture from the system to prevent freeze-ups and corrosion.

Spring brakes apply by spring force when air is released and release only when air pressure is restored.

The yellow diamond parking brake knob controls the spring brakes: pull to apply, push to release.

Static leak test: fully charged system, engine off, brakes released - more than 2 psi loss in one minute is a leak.

Applied leak test: hold the foot valve down for one minute - more than 3 psi loss in one minute is excessive.

Low-pressure warning check: with the engine off, pump the foot valve until the light and buzzer activate; they must come on before pressure falls below 60 psi.

Spring brake check: with the system charged, pull the yellow knob and fan the foot valve; the spring brakes must set by about 45 psi.

Compressor build-up: from 85 to 100 psi should take roughly 45 seconds at operating rpm (about 30-60 seconds is a common acceptable range).

Governor cut-out: the compressor should stop pumping near the governed pressure, typically about 125 psi.

Drain moisture from the reservoirs daily and check that the air dryer is working.

Walk-around inspection: look for cracked hoses, loose clamps, chafed lines, and audible leaks.

Slack adjuster check: pull each slack adjuster by hand; more than about 1 inch of free play at the push rod means adjustment is needed.

Push-rod stroke measures adjustment: too long a stroke means the chamber runs out of travel before full braking force reaches the drum.

Rule of thumb: with the brakes fully applied, push-rod travel should not exceed about 1 inch per 10 psi of application pressure (check the manufacturer's limit).

Manual slack adjusters can be adjusted, but automatic slack adjusters should not be manually adjusted to hide a chronic problem.

Warning devices: a low-pressure light and buzzer (below about 60 psi) and a parking brake indicator must be checked before driving.

A rapid loss of air pressure while driving, or pressure that will not build, means stop safely - the brakes may fail completely.

A dragging brake (hot drum, smoke, pull to one side) can come from a stuck slack adjuster, broken spring, or seized S-cam.

Brake fade from overheated drums reduces stopping power; use a lower gear and engine braking on long downgrades.

A compressor that never reaches cut-out, or a gauge that keeps climbing, points to a governor or safety-valve fault.

If the low-pressure warning sounds while driving, stop safely, set the parking brake, and check the system before continuing.

The parking brake is the spring brake: pulling the yellow knob releases air and lets the spring apply the brakes; it stays applied without air pressure.

Do not use the parking brake for a normal moving stop except in an emergency; use the foot valve for routine braking.

On a long downgrade, select a low gear before starting down and use engine braking; the foot brake is for control, not for holding speed continuously.

If the service brakes fail, apply the parking (spring) brake gradually, downshift, and look for a runaway ramp or safe escape route.

Stopping distance grows with speed and weight; an air-braked combination vehicle needs far more distance than a car, so increase following distance.

Before backing or parking, set the parking brake and chock the wheels on a grade; a spring brake can release accidentally if air is restored.

In cold weather, drain moisture and let pressure build fully before moving; frozen lines or valves can prevent release or application.

After a hard stop or long downgrade, let the brakes cool before parking; setting the parking brake on hot drums can cause cracking or fade.

When the low-air warning activates, stop as soon as it is safe, set the parking brake, and do not move until the system is repaired.

The compressor is engine-driven and supplies air to the reservoirs; it does not store air itself.

The governor controls compressor cut-in and cut-out pressure, typically unloading near 125 psi and resuming near 85 psi.

The safety valve on the supply reservoir vents at about 150 psi if the governor fails and pressure keeps rising.

The foot valve (treadle) is a graduated valve: more pedal travel admits more air and produces more braking force.

The supply reservoir feeds the primary and secondary circuits through one-way check valves, so a failed circuit cannot drain the other.

The alcohol evaporator or air dryer removes moisture from the system to prevent freeze-ups and corrosion.

Spring brakes apply by spring force when air is released and release only when air pressure is restored.

The yellow diamond parking brake knob controls the spring brakes: pull to apply, push to release.

Static leak test: fully charged system, engine off, brakes released - more than 2 psi loss in one minute is a leak.

Applied leak test: hold the foot valve down for one minute - more than 3 psi loss in one minute is excessive.

Low-pressure warning check: with the engine off, pump the foot valve until the light and buzzer activate; they must come on before pressure falls below 60 psi.

Spring brake check: with the system charged, pull the yellow knob and fan the foot valve; the spring brakes must set by about 45 psi.

Compressor build-up: from 85 to 100 psi should take roughly 45 seconds at operating rpm (about 30-60 seconds is a common acceptable range).

Governor cut-out: the compressor should stop pumping near the governed pressure, typically about 125 psi.

Drain moisture from the reservoirs daily and check that the air dryer is working.

Walk-around inspection: look for cracked hoses, loose clamps, chafed lines, and audible leaks.

Slack adjuster check: pull each slack adjuster by hand; more than about 1 inch of free play at the push rod means adjustment is needed.

Push-rod stroke measures adjustment: too long a stroke means the chamber runs out of travel before full braking force reaches the drum.

Rule of thumb: with the brakes fully applied, push-rod travel should not exceed about 1 inch per 10 psi of application pressure (check the manufacturer's limit).

Manual slack adjusters can be adjusted, but automatic slack adjusters should not be manually adjusted to hide a chronic problem.

Warning devices: a low-pressure light and buzzer (below about 60 psi) and a parking brake indicator must be checked before driving.

A rapid loss of air pressure while driving, or pressure that will not build, means stop safely - the brakes may fail completely.

A dragging brake (hot drum, smoke, pull to one side) can come from a stuck slack adjuster, broken spring, or seized S-cam.

Brake fade from overheated drums reduces stopping power; use a lower gear and engine braking on long downgrades.

A compressor that never reaches cut-out, or a gauge that keeps climbing, points to a governor or safety-valve fault.

If the low-pressure warning sounds while driving, stop safely, set the parking brake, and check the system before continuing.

The parking brake is the spring brake: pulling the yellow knob releases air and lets the spring apply the brakes; it stays applied without air pressure.

Do not use the parking brake for a normal moving stop except in an emergency; use the foot valve for routine braking.

On a long downgrade, select a low gear before starting down and use engine braking; the foot brake is for control, not for holding speed continuously.

If the service brakes fail, apply the parking (spring) brake gradually, downshift, and look for a runaway ramp or safe escape route.

Stopping distance grows with speed and weight; an air-braked combination vehicle needs far more distance than a car, so increase following distance.

Before backing or parking, set the parking brake and chock the wheels on a grade; a spring brake can release accidentally if air is restored.

In cold weather, drain moisture and let pressure build fully before moving; frozen lines or valves can prevent release or application.

After a hard stop or long downgrade, let the brakes cool before parking; setting the parking brake on hot drums can cause cracking or fade.

When the low-air warning activates, stop as soon as it is safe, set the parking brake, and do not move until the system is repaired.