Reading the air brake pressure gauge
Lesson 1 of 25 in CDL Air Brakes. It plays free, with no account.
The lesson, in words
You push the brake pedal to apply the brakes. Pushing harder applies more air pressure.
Air enters the brake chambers and pushes rods outward. Those rods move the parts that turn the brake camshaft.
Pressing and releasing the pedal unnecessarily can use air faster than the compressor replaces it. If pressure gets too low, your brakes won't work.
This gauge shows your supply pressure. A dual system shows each half, primary and secondary: two gauges, or one gauge with two needles. Watch both.
Start the engine and the compressor builds pressure. At about one twenty five the governor cuts out.
Use the brakes and pressure drops. Around one hundred psi the governor cuts back in and refills the tanks.
The low air warning must come on before pressure drops below…
- 100 psi
- 60 psi
- 20 psi
Answer: 60 psi. A warning, usually a red light, must come on before 60 psi. Stop and park safely as soon as you can. (Texas manual: 55.)
Keep losing air and the spring brakes come on between twenty and forty five psi, ready or not.
Next lesson: Three brake systems in one
The full class, $19 at launch.
Paid once, when it opens. Lesson 1 is free now.
- Animated systems
- Your state’s manual numbers
- Practice tests that explain
Not affiliated with FMCSA or any state DMV. Check your state’s CDL manual.