SEMI TRUCK TRACTOR AIR BRAKES

Well, here again I am going to blog about semi truck air brakes but, this time I am going to talk about the tractor brakes.

I would like to start off explaining the buttons on the dash. The red button is the (tractor protection, break away) valve. This valve controls the air to the red (emergency) air line to the trailer and has nothing to do with the tractor brakes. It fills a tank on the trailer with air at the pressure shown on the dash air gauge. This air pressure releases the trailer spring loaded parking brakes so that the trailer wheels are free to roll.

The yellow valve controls the spring parking brakes on the tractor and has nothing to do with the trailer. Each valve is independent of the other even though they are both together on the dash. When you pull the yellow valve it engages the tractor parking brakes. Unless there have been some changes that I don't know about you can push each valve in independently one at a time or you can push them both in at once. If you push them both in then that will release both the tractor and the trailer parking brakes.

When you charge the trailer system with air then the red emergency line is pressurized and is kept that way all the time the red valve is pushed in. The blue (service) line has no air in it until you either push the foot valve or pull the trailer hand valve. The pressure in the blue line will only have as much pressure as what is being used to apply the trailer brakes. A truck does not release any air in any line to apply the trailer brakes. It is all done with pressure from the tractor and the tank on the trailer once the valve on the trailer tank is actuated. When you pull the trailer hand valve that only operates the trailer brakes. The only way to operate all the brakes on the vehicle is to use the foot valve.

Never use the hand valve as a parking brake. Pull the red emergency and the yellow tractor parking brake valves on the dash whenever you wish to park no matter how short a time that may be. Never use just the trailer brakes to stop a vehicle as that will heat up the brakes on the trailer and possibly cause other problems.

If you have the trailer emergency line charged and you remove the glad hand connecting to the trailer then you will lose air in that line and the trailer spring brakes and the service brakes which have no spring brakes on them will apply. When the emergency line loses pressure then the valve on the trailer tank allows air from the tank to apply the brakes and that will keep the brakes applied until that tank loses air pressure then those without spring brakes will release. The spring brakes will remain applied even though the trailer tank loses air because they are spring loaded and the spring pressure will keep the brakes applied. The valve will not allow any air to be removed from the tank except that which applies the brakes. It puts pressure into the tank but the valve is one way only and won't let any air come back out of the line. If, after you remove the glad hand, you should happen to hear air coming out of the line then the valve is defective and needs to be replaced.

I realize that I said I was going to talk about tractor brakes and spoke a lot about the trailer. That is because they are both related and are both operated from the tractor so I can hardly speak of one without speaking of the other.

I am going to publish this blog now even though I may add a bit more to it later.

If you would like more information on how brakes work from the people who make the parts for the brakes then go.to this link. http://forums.aths.org/Attachment20808.aspx You will find the Bendix Air Brake Handbook. It explains it the best of what I have found on the web. The description starts on page 5. It will take a bit to load because it is 60 some pages.

Donald Fout
mrdon636@aol.com