Originally Posted by lefaivre
The compressor should not cut in if the pressure is between 100 and 130 lbs. Once.the pressure drops below 100 the compressor will cut in and bring the system back up to high pressure cutoff at 130 lbs. When this happens you should hear air vent
.... just like on a city bus
All air brake systems are the sames although the absolute pressure will vary.
Actually it works like this: The compressor runs CONTINOUSLY, always increasing the pressure in the tanks. That is, the compressor is a DIRECT DRIVE to the engine; there is no "clutch". To keep the tanks from exploding, a device referred to as a "governor" releases EXCESS pressure every couple of minutes.
The governor "popping off" is the sound you are hearing. It will release (as needed) whether you are moving, in park, stopped at a light, stepping on the brake, NOT stepping on the brake, whatever. The ONLY thing necessary for the governor to release every two minutes or so, is that the engine be running.
Think of it as similar to your wife's pressure cooker as it sits on top of the stove. When the pressure gets to high, the safety valve will open temporarily to release SOME of the pressure, then close to start the cycle again.
I rarely look at my pressure gauges. You have two of them because you really have (at least some) brake system parts "duplexed" for safety. Just like you car has two separate (and isolated from each other) brake systems for the front and rear axles.
If your pressure ever drops below around 75 lbs you'll get an audible alert. If you choose to ignore THAT, then at around 40 lbs, the yellow knob will automatically pop out, effectively applying your parking brake for you, whether you like it or not!!!
You can test this by stopping the engine, leave the parking brake off and then (while on level ground) repeatedly "pump" the brake pedal to deplete the pressure in the tanks. At around 40 lbs, the parking brake will set itself.
FWIW, a commercial drivers license test requires that you demonstrate your knowledge of this air brake feature, as well as quite a bit of other info about the air brakes. Even though it's rarely done in "real life", part of a standard pre-trip inspection on an 18 wheeler (required by law) is a brake test that takes about 2-3 minutes to complete and checks for SEVERAL brake related functions.