Commercial Driver License Study Group

2012

Section 5 – part a

Air Brakes

Air brake systems

• Service brake

• Parking brake

• Emergency brake

Parts of air brake system

• Air compressor

• Air compressor governor

• Air storage tanks

• Air tank drains

• Alcohol evaporator

• Safety valve, 150 psi

Parts of air brake system

• Brake pedal

• Foundation brakes

– Brake drums, shoes, linings

– S-cam brakes

– Wedge brakes

– Disc brakes

Parts of air brake system

• Supply pressure gauges

• Application pressure gauge

• Low air pressure warning – before 60 psi

• Stop light switch

• Front brake limiting valve

Parts of air brake system

• Spring brakes

– Emergency & parking brakes

– Held on by mechanical force

– Air pressure holds them back when driving

– Applied when air pressure drops to 20 to 45 psi

– Slow vehicle & park when you hear/see warning at 60 psi

Parts of air brake systems

• Parking brake controls

– Diamond-shaped, yellow knob, out=ON

– Caution ! Never push brake pedal when spring breaks are on = too much force

– Modulating control valve

• Variable application of spring brakes

– Dual parking control valves

• Separate backup air to move vehicle in emergency

Anti-lock braking system (ABS)

• Activates when wheels about to lock up

• Helps keep vehicle under control

• Does NOT decrease or increase normal braking

• Required after 1997/98

• Yellow malfunction lamps on tractor/trailer

Dual air brakes

• Dual air brake

– Primary -- Rear axle

– Secondary -- Front axle

• Build up air pressure to 100 psi

– Warning light/buzzer at 60 psi or less

Inspecting air brake systems

• Fit into 7-step inspection method

• Engine compartment – air compressor drive belt

• Walkaround

– slack adjusters on s-cam brakes – 1 inch

– Brake drums, linings, hoses

Step 7 – final air brake check

• Low pressure warning signal

– Engine off when enough air pressure so low pressure warning signal is NOT on

– Turn on electrical power

– Step on/off brake pedal to reduce air tank pressure

– Low air pressure warning signal must come on before pressure less than 60 psi

Step 7 – final air brake check

• Spring brakes come on automatically

– Continue to fan off air pressure by stepping on/off brake pedal

– Tractor protection valve and parking brake valve should close (pop out) on tractor-trailer

– Parking break valve should close (pop out) on other vehicle types

– When air pressure falls to 20 to 40 psi

– Spring brakes come on

Warning at 60 psi

Spring brakes on at 20 to 40 psi

Step 7 – final air brake check

• Rate of air pressure buildup

– At operating rpms, pressure should build from 85 to 100 psi within 45 seconds in dual air systems

– Manufacturer’s specifications

– Single air systems (pre-1975) pressure should build from 50 to 90 psi within 3 minutes with engine at idling speed of 600-900 rpms.

Step 7 – final air brake check

• Air leakage rate

– With 125 psi fully-charged air system

– Turn off engine, release parking brake, time drop

– Single vehicle = <2 psi/minute

– Combination vehicle = <3 psi/minute

Step 7 – final air brake check

• Air leakage rate, continued

– THEN apply 90 psi or more with brake pedal

– Single vehicle – if more than 3 psi/min

– Combination vehicle – if more than 4 psi/min

– CHECK FOR LEAKS & FIX BEFORE DRIVING

Step 7 – final air brake check

• Air compressor governor cut-in / cut-out

– Run engine at fast idle

– Governor cut-out about 125 psi

– Step on and off brake to reduce air pressure

– Compressor governor cut-in about 100 psi

– Manufacturer’s specifications may be different

Step 7 – final air brake check

• Test parking brake

– Stop vehicle, put parking brake on

– Gently pull against it in a low gear

• Test service brakes

– Wait for normal air pressure

– Release parking brake

– Move forward 5mph, apply brakes

Using air brakes

• Normal stops

• With Anti-lock Brakes

– Tractor, trailer, one axle

– Brake normally

• Use only force necessary for safe stop

• Brake in same way

• May have to back off brakes to stay in control

– Gives you ability to steer around obstacle

Using air brakes

• Emergency stops – Brake to keep vehicle in straight line

– Controlled braking • As hard as possible WITHOUT locking wheels

• Keep steering movements small

• If wheels lock, or you need to steer, release/reapply

– Stab braking • Apply brakes all the way

• Release when wheels lock up

• As wheels roll, apply brakes full again

Stopping Distance

• Air brakes = added delay for time required for the brakes to work after the brake pedal is pushed (1/2 sec or more for air to travel through lines).

• Perception + Reaction + Brake lag + Braking = Total Stopping Distance

Brake fading/failure

• Excessive heat can cause brake failure

– Chemical changes = reduced friction

– Expansion of brake drums = reduced friction

• Out of adjustment

Proper braking technique

• On a long downgrade brakes are a supplement to the braking effect of the engine. Once in proper low gear:

– Apply brakes just hard enough to feel slowdown

– Reduce speed to 5 mph below safe speed, release

– When speed increases to safe speed, repeat

Low air pressure

• If low air pressure warning comes on, stop and park

• Controlled braking available a short while

• Spring brakes at 20-45 psi

• Life get interesting

Parking brakes

• Every time you park

• Control knob – out to apply

• Drain air tanks each day

• Exceptions:

– Don’t use when brakes are very hot

– Don’t use when brakes are very wet & freezing

– Use wheel chocks instead