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Braking Bad

By on December 30, 2011
RoadKing Mag

My hope for all drivers is that they will never experience how emergency brake systems work. But if the need arises, it’s good to be aware of what happens to make those systems function at peak level.

Going down the road, the typical driver is unaware of the emergency brake systems. They are idle, and though they play a small part in regular brake function, for the most part the system is just along for the ride.

The tractor’s primary braking system is the rear brakes, responsible for the majority of the braking on the vehicle. This system has a protection device to make sure that if there is a catastrophic failure the brakes can still be engaged for safe navigation off the road. For example, a truck may roll over an object in the road and rip a hole in the air supply side of the rear braking system. Many OEMs use a Bendix valve called an SR7 to come to the rescue.

This valve will take air pressure that is designed to operate the front brakes and divert some of that air to the rear brakes, so the driver still has some control.

But in order to optimize this emergency braking function, the air system must be maintained in two very important ways.

Air volume in the tanks

Air pressure is one thing and air volume is totally another. Drivers need to drain their truck’s air tanks regularly to maintain air volume. Why? If air tanks are filled to 50 percent capacity with water and contaminants, the truck gauges will show adequate air pressure, but the volume of the air that is needed to operate those rear brakes is cut by 50 percent. A driver won’t know that there is not enough air volume until an emergency situation arises and the system automatically diverts that air from front brakes to rear brakes. And then you’ve got a catastrophic situation.

That is a compelling reason to get the water and contaminants out of the air tanks every day. But it’s common to find that this simple procedure is not being done because the pull cord on the truck is not functional, and the driver may not realize the sense of urgency to fix it because the vehicle operates properly.

But oh my goodness, if an emergency arises, such as a loss of primary air, it’s too late and the outcome could be serious.

A working air dryer is vital

I encourage truckers to maintain their vehicle’s air tanks by doing something really simple: Pay attention to the air dryer.

One of the main things to do is to change the desiccant cartridge in the air dryer. That is a filter cartridge and it has a maintenance interval that should be followed and performed by a qualified service facility. Proper maintenance of this component is important because it keeps moisture and contaminants out of the air system.

Too often, when a driver has an air leak and goes in for service, they will ask that the system be bypassed so they can get back on the road quickly. This is a common practice, but it is also meant to be a temporary fix until proper repairs can be made.

Drivers need to understand the urgency of making that repair. It is critical, not only to having the proper air volume in an emergency braking situation, but to the whole air system. A bypassed air dryer damages critical components, like relay valves, because contaminants can circulate through the air system. That does long-term damage to the braking system.

Unfortunately, many drivers have good intentions, but once the bypass is done and the vehicle operates, it’s a case of “out of sight, out of mind.”

Keep the tractor protected

It’s also important to maintain the tractor protection system. If a trailer disconnects from its truck during operation, the tractor protection system helps the trailer brakes lock up so that the trailer won’t run into the tractor. It also protects air pressure in the truck, so when the air lines between the two sections get disconnected, there is enough air pressure remaining in the tractor to keep the brakes working. This tractor protection system must be tested properly on a scheduled basis.

The federally required annual inspection should consist of a good evaluation of your tractor protection system, and it is important to use a qualified inspector and service facility to be certain they are properly testing that system.

Homer Hogg , Maintenance Supervisor for TA and Petro, has worked as a truck technician for 30 years. He is a Daimler Certified Trainer, Ryder Master Technician and a member of the Nashville Auto Diesel College Hall of Fame.

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One Comment


    January 12, 2012 at 11:34 am


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