Important Numbers
This is the governor cut-in pressure where the compressor begins pumping air.
What is 100 PSI?
Maximum static air loss for a single vehicle.
What is 2 PSI per minute?
This device pumps compressed air into the air tanks.
What is the air compressor?
The most common type of air brake.
What is the S-cam brake?
This warning can be a red light, buzzer, or wig-wag.
What is the low-air warning?
This pressure tells the governor to stop the compressor.
What is 125 PSI (cut-out)?
Maximum static air loss for a combination vehicle.
What is 3 PSI per minute?
This controls when the compressor starts and stops pumping.
What is the governor?
This pushes the brake shoes apart inside an S-cam brake.
What is the S-cam?
This mechanical device drops into the driver's view when air pressure is low.
What is a wig-wag?
The safety valve normally opens at this pressure.
What is 150 PSI?
Maximum applied air loss for a single vehicle.
What is 3 PSI per minute?
This stores compressed air for the braking system.
What are the air storage tanks?
These press against the drum to create friction.
What are the brake shoes and linings?
Large buses often activate their low-air warning around this pressure.
What is 80–85 PSI?
The low-air warning must activate before pressure drops below this number.
What is 55 PSI?
Maximum applied air loss for a combination vehicle.
What is 4 PSI per minute?
This removes water and oil from the air tanks.
What are the air tank drains?
This type of brake uses a wedge instead of an S-cam.
What are wedge brakes?
If the low-air warning comes on while driving, this is what you should do.
What is stop and safely park as soon as possible?
Spring brakes normally apply automatically within this pressure range.
What is 20–45 PSI?
The memory trick for leakage limits.
What is 2/3 without the pedal, 3/4 with the pedal?
This adds alcohol to reduce ice forming inside the air brake system.
What is the alcohol evaporator?
This type of brake clamps a rotor between pads.
What are disc brakes?
If the safety valve releases air, this should happen before driving again.
What is have the system repaired?