MA CDL Air Brakes cheat sheet

Air path: compressor → governor-controlled supply → air dryer → reservoir tanks → foot valve (brake valve) → brake chambers → slack adjusters → foundation brakes.

The compressor is engine-driven and feeds the system; the governor cuts compressor output in at roughly 100 psi and cuts it out near 120-125 psi, maintaining a working range.

The air dryer removes moisture and oil before air enters the reservoirs; a saturated or failed dryer carries water into tanks and valves and can cause freeze-ups in cold weather.

The foot valve (treadle) is a graduated control: the further it is pushed, the higher the delivery pressure to the brake chambers, so braking force is proportional to treadle travel.

The spring (parking) brake chamber applies mechanically when air pressure is released; system pressure must be above about 45 psi (typically 60-75 psi to fully release) before the spring brakes will release.

The safety valve on the supply reservoir protects the system from over-pressurization, typically opening around 150 psi; it is not a regulator for normal operation.

The low-pressure warning device (buzzer and/or light) must activate before pressure drops below about 60 psi; on vehicles built after the relevant rule date a wig-wag brake lamp is also required.

The slack adjuster converts pushrod stroke into rotation of the brake cam or S-cam; excessive pushrod stroke means the brakes are out of adjustment and stopping distance increases.

A dual (split) system has two separate circuits, typically a primary circuit for the rear brakes and a secondary circuit for the front brakes, each with its own reservoir.

The two circuits are isolated by check valves so a failure or leak in one circuit does not drain the other; the intact circuit still provides braking.

A failed circuit is signaled by a pressure drop on one gauge, illumination of the low-pressure warning, and the spring brakes applying when supply pressure falls below the release threshold.

The tractor protection valve closes automatically when tractor supply pressure drops (typically below about 45 psi), preventing loss of air from the tractor if the trailer separates.

The trailer supply valve (red knob) and tractor parking valve (yellow knob) are push-pull controls; the red knob pops out automatically on pressure loss to signal trailer breakaway protection.

The front brake limiting valve (where installed) reduces front braking force in slippery conditions; it must be in the normal position for dry-road stopping.

The alcohol evaporator or air dryer protects against moisture; alcohol is not a substitute for a functioning dryer and should not be added to a system with a dryer that specifies otherwise.

Dual-circuit systems require that both circuits build pressure; a slow build on one gauge indicates a restriction or leak in that circuit that must be repaired before driving.

With the engine off and the system fully charged, listen for leaks; a steady hiss indicates an open leak, and the system should hold pressure with no audible leakage.

Check that the low-pressure warning buzzer and light come on when pressure is below about 60 psi, then build pressure and confirm they go off.

Check the compressor build rate: with the engine at operating rpm, pressure should rise from about 85 to 100 psi within a specified time (commonly about 45 seconds for a single unit).

Check the governor cut-out and cut-in: compressor should stop near 120-125 psi and restart near 100 psi; readings outside this range indicate governor or compressor problems.

Inspect slack adjusters and pushrod stroke against the manufacturer's limit; a stroke beyond the limit means the brakes need adjustment before the vehicle is driven.

Check brake chamber pushrods for a bent, broken, or missing pushrod, cracked chamber, and loose clamps; these are out-of-service defects.

Drain all reservoirs (supply and both circuits) of moisture and check for oil or excessive water; a milky or oily discharge indicates compressor or dryer problems.

Check that spring brakes hold the vehicle: with the parking brake set, the vehicle should not roll; release only after pressure is above the release threshold.

Static leak test: charge the system to governor cut-out, shut off the engine, and with the brakes released observe pressure loss; allowable loss is typically no more than about 2 psi per minute (single unit) or 3 psi per minute (combination).

Applied leak test: with the system charged and the engine off, hold the foot valve fully applied for one minute; allowable loss is typically no more than about 3 psi per minute (single unit) or 4 psi per minute (combination).

Low-pressure warning test: with the engine off, fan or pump the brakes to reduce pressure; the warning must sound before pressure falls below about 60 psi.

Spring brake test: with pressure below about 45 psi, the spring brakes should apply automatically; the vehicle must not be able to move.

Compressor build test: with the engine running at fast idle, time the rise from 85 to 100 psi; excessive time indicates a weak compressor, slipping belt, or leak.

Governor test: note the pressure at which the compressor cuts out and cuts back in; a cut-out well above 125 psi or a cut-in well below 100 psi indicates a faulty governor.

Any leak that produces an audible hiss, or pressure loss beyond the allowable rate, is a defect requiring repair before the vehicle is placed in service.

Perform leak tests with the engine off and the system at full pressure; testing with the engine running masks leaks because the compressor keeps replenishing the system.

Apply the brakes with a firm, steady push and hold; fanning or pumping the treadle on air brakes wastes air and can reduce available braking pressure.

Air brake lag means braking begins slightly after the treadle is applied; allow increased following distance compared with hydraulic brakes.

Brake application pressure should be enough to control speed without locking wheels; a locked wheel skids and loses steering control.

Stopping distance increases with speed squared and with load; a heavier vehicle at higher speed needs disproportionately more distance.

Use the brakes in short, firm applications to control speed, allowing time for the system to recover pressure between applications.

Watch the air pressure gauges while driving; a continuous drop in pressure indicates a leak or excessive brake use and requires stopping to investigate.

The parking (spring) brakes must never be used for normal stopping; they apply fully and can cause a skid or loss of control.

Before descending a grade, select a gear low enough that engine braking controls speed and the service brakes are used only for trimming speed.

Select the gear before starting down a grade; a gear that holds the vehicle without braking is the correct choice, and shifting down on the grade may be impossible.

Use engine braking (and retarder where equipped) as the primary speed control on a downgrade; the service brakes are for supplemental speed reduction.

Continuous light brake application on a long grade overheats the drums and shoes, causing brake fade and loss of stopping power.

If brakes fade, do not continue to apply harder; reduce speed by gearing down, and use an escape ramp or safe stopping area if speed cannot be controlled.

A stab (snub) brake application - firm and brief - slows the vehicle and then releases, allowing the brakes to cool between applications.

In an emergency stop with air brakes, apply the foot valve fully and hold it; do not pump the pedal. If the system fails, the spring brakes apply automatically as pressure drops.

The emergency (spring) brake can be applied by pulling the parking control; it applies full braking force and should be used only when service brakes fail or to secure a stopped vehicle.

On a downgrade, check the mirrors and gauges; rising speed with full engine braking and normal brake application means the grade is too steep for the selected gear - slow down immediately.

Low air pressure warning: a buzzer and/or light must activate before pressure drops below about 60 psi; if it fails to activate, the vehicle is out of service.

The wig-wag (swinging) brake light warns following drivers of aggressive braking; it is required on vehicles built after the applicable rule date and is a warning device, not a brake control.

A compressor that fails to build pressure or builds too slowly indicates a worn compressor, slipping or broken belt, or a major leak; the vehicle must not be driven.

A governor stuck in the cut-in position causes pressure to keep rising and the safety valve to release; this is an over-pressurization defect requiring repair.

A failed air dryer or saturated desiccant allows moisture into the system, causing frozen valves in winter and corrosion; drain reservoirs and service the dryer.

If the low-pressure warning sounds while driving, stop as soon as it is safe: pressure loss will eventually apply the spring brakes and the vehicle will stop on its own.

A leaking brake chamber or diaphragm causes a constant air loss and weak braking on that wheel; it shows up as pressure loss during the applied leak test.

If the spring brakes apply unexpectedly while driving, the cause is a loss of air pressure; stop, find and repair the leak, and do not attempt to drive with the system below the release threshold.

Out-of-service defects include: low-pressure warning inoperative, air loss exceeding the allowable rate, compressor unable to build pressure to at least 85 psi, and spring brakes that do not apply automatically.

The low-pressure indicator must be checked during the pre-trip inspection by reducing pressure and confirming the buzzer and light operate before 60 psi.

An inoperative wig-wag brake light is a defect on vehicles where it is required; it is checked by applying the brakes and observing the lamp.

A safety valve that releases during normal operation indicates a faulty governor or over-pressurization; the vehicle should be removed from service until repaired.

Any audible air leak, cracked brake chamber, or pushrod stroke beyond the adjustment limit is an out-of-service condition.

The parking brake must hold the vehicle on a grade; failure to hold indicates weak spring brakes or a linkage defect and is an out-of-service condition.

A vehicle that cannot maintain at least 85 psi with the engine running has a compressor or leak defect and must not be driven.

Warning devices are diagnostic: a warning that activates during normal driving signals a developing fault, and the driver should stop and investigate rather than continue.

If the service brakes fail, apply the parking (spring) brake control gradually rather than all at once to avoid locking the wheels and losing steering control.

Use an escape ramp when speed cannot be controlled on a downgrade; aim for the center of the ramp and expect a soft, decelerating surface.

If a tire blows, hold the steering wheel firmly, avoid hard braking, and slow gradually before pulling off the road.

On a downgrade, if the vehicle speed continues to rise despite gearing down and braking, brake firmly and look for an escape route rather than riding the brakes.

Emergency braking on a split-friction surface (ice on one side) requires gentle, steady pressure; hard braking can cause a spin.

After any emergency stop, check the air pressure gauges and brake function before resuming; an emergency application may have depleted pressure.

The spring brakes apply automatically when pressure drops below about 45 psi; this is the system's built-in emergency stopping feature and should not be relied on for normal stopping.

A runaway vehicle on a downgrade should be steered toward a safe runoff area; steering control is maintained only if the wheels are not locked.