Air brakes Flashcards
3 types of braking systems
Service, Parking, & Emergency
Air compressor
Pumps air into the storage tanks
Governor
Controls when the air compressor will pump air into the air storage tanks
Cut-out level
When air tank pressure rises to around 125 psi, or the cut-out level, the governor stops the compressor from pumping air
Cut-in level
When air tank pressure falls to around 100 psi, or the cut-in level, the governor allows the compressor to start pumping air again
Safety relief valve
Valve is usually set to open at 150 psi (if valve releases air, something is wrong)
Parts of the brake
To stop, the brake shoes and linings are pushed against the inside of the drum. This causes friction, which slows the vehicle
S-cam brakes
When the pedal is pushed, air is let into each brake chamber. Air pressure pushes the rod out, moving the slack adjuster, thus twisting the brake camshaft (and turning the s-cam). The s-cam forces the brake shoes away from one another and presses them against the inside of the brake drum
Supply pressure gauge
Tell you how much pressure is in the tank
Application pressure gauge
Shows how much air pressure you are applying to the brakes
Low air pressure warning signal
A warning signal that you can see that must come on before the air pressure in the tanks falls below 60 psi (can be a red light, buzzer, or a wig wag)
Front brake limiting valve
Valve that cuts the air pressure in the front brakes by half (on vehicles made before 1975)
Spring brakes
Powerful springs held back by air pressure
Spring brakes on tractor and straight trucks
Will come fully on when air pressure drops to a range of 20 to 40 psi
Parking brake
You pull the diamond-shaped, yellow knob to put on the parking brakes and push it in to release them (older vehicles may have a parking brake that is controlled by a lever.)
Modulating control valves
A control handle on the dashboard may be used to apply the spring brakes gradually
Antilock Braking System (ABS)
A computerized system that keeps your wheels from locking up during hard brake applications. (required on truck tractors built on or after March 1, 1997 and other air brake vehicles built on or after March 1, 1998)
Dual Air Brake
Most heavy-duty vehicles use dual air brake
systems for safety. A dual air brake system has
two separate air brake systems, which use a single
set of brake controls. Each system has its own air
tanks, hoses, lines, etc. One system typically
operates the regular brakes on the rear axle or
axles. The other system operates the regular
brakes on the front axle
PSI in dual air system
Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems
Warning light (systems with dual air)
The warning light and buzzer should shut off when air pressure in both systems rises to the value set by the manufacturer. (The value must be greater than 60 psi.)
Slack adjusters on s-cam brakes
Park on
level ground and chock the wheels to prevent the
vehicle from moving. Release the parking brakes
so you can move the slack adjusters. Use gloves
and pull hard on each slack adjuster that you can
reach. If a slack adjuster moves more than about
one inch where the push rod attaches to it, it
probably needs adjustment.
Brake drums
Must not have cracks longer than one half the width of the friction area
Air pressure build up
Pressure should build from 85 to 100 psi within 45 seconds in dual air systems/single air systems, typical requirements are pressure buildup from 50 to 90 psi within 3 minutes
Controlled braking
You apply the brakes as hard as you can without locking the wheels
