semi-truck trailer air brake system

OK Before I start I would like to say that I don't know anything about blogging. I was told that I should do this just so I can get the hang of it. The things that I would like to talk about the many and varied. I would like to talk about trucks and their safety features and many other things pertaining to trucks since I was a over-the-road truck driver most of my life. I am also known as jack of all trades. They also call me a tinker because I like to take things apart that don't work and make them work even if I don't need them just to see if I can.

The first thing I am going to talk about is truck brakes since there seems to be quite a misconception as to how they work. I have been reading some articles on the web lately which are supposed to be explaining how they work and they are not correct and yet these articles are read by many people. I would like to set the record straight and get people to understand how they do work. Let me explain that I am not too well versed on all the advancements made in the braking systems of today but as far as I know they still work in the same manner as the older brakes with some improvements.

Many people are under the misconception that when you put air pressure into the lines that releases the brakes. To make the brakes apply you release pressure out of that line. That is not correct unless you are talking about only the spring loaded parking brakes. The service brakes are a whole different matter and are a separate system from the parking brakes.

When you push in the "Emergency" "Tractor Protection" valve on the dash that does pressurize the line called the emergency line and it does fill a tank on the trailer . This line does does not release any brakes other than the parking brake which is a spring loaded brake and the air pressure is used to compress a spring to release those brakes. The air in the tank is used to apply the service brake but only when the air in the emergency line is expelled or released. That is so that in the event that you should have the trailer separate from the tractor and the emergency line should come apart then the trailer brakes will apply stopping the trailer even without any spring loaded brakes. The reason I say that is because many units have spring parking brakes on only one axle and not on both.

If you pull out the parking brake valve on the dash that only applies the parking brakes on the tractor. It has nothing to do with the parking brakes on the trailer. So, if you want maximum braking power while parked then pull both valves. That will apply the parking brakes on both the tractor and the trailer and until they leak off will apply air to the service brakes on the trailer if they are not equipped with spring parking brakes.

Maybe I should break this apart as well as I can because it may seem confusing to some so, I will explain the spring loaded parking brake on the trailer and then I will explain the service brake on the trailer. It is sometimes hard for a person who knows about something to explain it to someone who does not because the person who does not cannot understand some of the things that it person who knows says.

When it comes to the spring loaded parking brakes on a trailer. Air pressure is applied to the diaphragm inside the canister to push against the spring thereby releasing the brake. When there is no air against the diaphragm the spring pushes upon the rod which goes through the bottom of the chamber to push against the service brake diaphragm the same way as air pressure when you apply the brakes from the tractor but, at this time there is no air pressure against the service brake diaphragm, only the pressure of the spring pushing against the rod pushing the diaphragm down to apply the brake. When you again apply pressure to the spring brake diaphragm it collapses the spring thereby releasing the brake.

When it comes to the service brakes on a trailer. Remembering that the spring brake is now released because there is air pressure applied to that diaphragm. This air is contained in the spring brake canister and is separated from the service side by o-rings on the shaft from the spring brake. At this time all brakes are released and the wheel can roll freely.

When you apply pressure to the service line which is the line from the foot valve on the tractor then air is applied to the service brake diaphragm and pushes the push rod connected to the slack adjuster thereby applying the brakes. This air is separated from the air in the spring brake canister by the same o-rings on the spring brake shaft. When you release the foot brake then you hear the air being expelled through the exhaust port but only the air from the service brake canister and the brakes are released.

That is enough for this blog. I still have to explain the brakes on the tractor and how they are the same and yet different in some ways.