Air brake supply and service

by Ryan

Hi Jimmy and Hervy (spell your name right this time Hervy!)

Ryan here again with another question this time about the air brakes.

I'm trying to understand them to the best of my ability so I guess I explain what I think is happening in the air system and then you can correct me if I'm incorrect so I guess that will be my question - did i explain it correctly?

So the Service brakes gets compressed air from both the Primary Tank and the Secondary tank when you apply the brake pedal. The Secondary/Supply air tank is constantly giving air to the Spring Brake in order to "cage" the spring for both the tractor and trailer. When the air tank suffers air loss and the air gauge reads 60psi a warning light/buzzer will go off because the supply tank as lost too much air to compress the spring in the Spring Brakes. And the driver should pull over ASAP and when it is safe to do so.

So now with the two push/pull knobs inside the truck (this is where I'm confused, a bit). The yellow one is for? And the red one is for the trailer which looks like a stop sign in its shape. So is the Red knob just strictly for the trailer parking/Spring Brake and if so I don't really get what the Yellow knob is for 'cause doesn't that affect the trailer spring brakes too?

I hope you can help and explain it a bit better so that I can understand the air brake system.


P.S. I start my course tomorrow!!! I little nervous and excited at the same time!!! :)

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May 22, 2009
Ryan has more questions about trucking
by: Jimmy

Darn, Ryan, you made me have to put on my thinking cap for this one. I'll try to keep it simple, but I may confuse you even more.

All heavy trucks have air brakes. There is a compressor that runs off the engine when the engine is running to supply the air as it empties out of the tanks.A normal PSI is about 120 lbs, when the PSI drops down to 60 lbs, warning lights/buzzers go off to let the driver know he/she is low on air. The brakes won't lock up until you are down to about 35-40 lbs. If you maintained 60 lbs, you could keep going, but you need to know that as soon as you apply the brakes, you use 12-15 lbs and Dude, that is damn close to gettin' them locked up.

Many times, if you see that you are losing air, you can stop and find the leak and correct it. Could be a gladhand washer, a loose airline connection, even a tank bleed valve that is open. You need to have air supply open to attempt to find/hear the air leak. Sometimes you may need assistance, one person holds down the brake peddle and the other searches.

Now the yellow/red knobs on the dash. When you hook up power unit to trailer, you connect the air lines. Until you push the red knob IN, there is no air supply to the trailer. Red knob only supplies air to the trailer. The yellow knob supplies air to the power unit only. When you park, you can either pull both knobs out or just the yellow knob. If you just pull the yellow knob, you will set only the truck brakes. 99.9% of the time, tractor brakes only are enough to hold it. You'll notice I have referred to the truck tractor as a power unit, truck, and tractor. Most guys call it the tractor.

Now in parking on a hill, loaded, be sure and set truck and trailer brakes both. Parking in snowy weather overnight, just the tractor brakes. I've been in 30 below weather where I didn't set the brakes, instead I blocked the tires, so in the morning I didn't have to worry about the brakes being frozen.

So give 'em hell at school, Ryan, keep us posted. Jimmy P.S. Forgot to mention, know your brakes, they can save your life of get you killed. Don't ever skip over them when you pre-trip.

May 22, 2009
by: Anonymous

When operating bobtail only the yellow(tractor)valve is used. Push the valve in to release the tractor brakes. You already know more than most so you can relax now. Good Luck

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