Air Backfeeding From Trailer Emergency Supply Lines – Causes

A trailer emergency supply line carries air to fill the trailer’s tanks. These lines transport the air pressure utilized to operate trailer brakes.

Also referred to as supply or emergency lines, these lines must be protected in order to work efficiently. If not, they may compromise your trailer when it comes to braking.

Why is air backfeeding from my trailer emergency supply lines

So, if you notice that air is back-feeding from your trailer emergency supply lines, you have to detect the issue and diagnose it as soon as possible. In this post, we’re going to pinpoint why air is back-feeding from your trailer emergency line and what to do about it.

A Bad Air Brake Relay Valve

The work of the air brake relay valve is to remotely control the brakes at the rear of a semi-trailer in a tractor-trailer combination or a heavy truck. The friction applied by this valve slows and stops the brake drum’s turning, which eventually stops the wheel.

However, if the air brake relay valve is bad, it can cause many problems to the braking system. And one of those problems is leaking. First of all, a faulty air brake relay valve can cause air to back-feed in the supply line, which causes air to leak.

How To Fix It

You will have to detect which valve is back-feeding air. Afterward, you can also fix the leaking valves. But if the air brake relay valve is bad, you may have to replace it with a new one.

This can be pretty challenging to discover, and that’s why you may require the services of a professional mechanic to handle the issue amicably. But if you stop the leakage and air back-feeding, you can still use the valve.

Bad Tractor Protection Valve

A tractor protection valve is meant to protect the tractor air brake system if there is a trailer breakaway or a severe leak. Their main work is to prevent air loss from the towing vehicle.

Therefore, it shuts off air to the trailer before disconnecting the lines in an emergency. So, when the trailer supply line closes, the tractor protection valve on the towing truck also closes. On top of that, it prevents air loss out of the towing truck if there is no trailer connected.

But like all valves connected to the brake system, the tractor protection valve can also cause issues if it’s faulty. It can make air leak or back-feed. If you have such an issue, visit a mechanic to inspect your ar.

Note that the tractor protection valve is controlled by the trailer air supply control valve. It enables you to either open or close the tractor protection valve. Therefore, when the trailer supply valve is opened to permit air to the trailer, air pressure is also channeled to open the tractor protection valve and permit air to the trailer service line.

On the contrary, if the valve closes, it ceases any air from getting out and allows air out of the trailer supply line that makes the trailer emergency brakes engage. Air can also back-feed when the valve closes.

How To Fix It

If you don’t want to have issues while applying emergency brakes, make sure you replace a bad tractor protection valve. If air back-feeds or leaks from the emergency line, the breaks won’t engage as needed.

Visit a professional mechanic to fix this problem as the tractor protection valve can be quite challenging to fix. Also, you should check the supply control valve and ensure it’s also working fine.

Bad Spring Brake Valve

The spring brake valves were created to perform three main functions: to release the spring brake side of the brake chamber, fill the reservoirs, and engage the spring brakes for parking and during emergency situations like a breakaway.

The spring brake operates in a simple way. The supply port on the valve is linked to an air line by a red coupler. So, when air is introduced, it starts to fill the spring brake chambers via the delivery ports (emergency line and service line).

Finally, the pressure forms until it cage the spring; thus, permitting the trailer to move. The reservoirs are also filled at the same moment.

When the pressure is dispatched, the valve dispatches the air from the spring side of the chambers. Hence, this locks the brakes. On the contrary, the air in the reservoir is trapped by a check valve.

Air backfeeding from trailer emergency supply lines causes

If this valve is bad, it will cause the air to back-feed through the red port, which is the emergency supply line. Also, the spring brakes won’t work effectively when it comes to parking or during a breakaway.

How To Fix It

Start by inspecting the spring brake valve. If it’s not in good condition, get a professional mechanic to replace it. A bad spring brake valve can cause a leak or air to back-feed depending on where the damage happened.

Wrongly Matched Trailer Lines

When connecting a trailer and tractor that’s towing it, you need to make two vital connections for the brake system to function correctly. The first connection transports air to the trailer’s tank and is referred to as an emergency or supply line. The other connection transports pressure for the service brakes and is referred to as the service line.

The emergency line can easily be differentiated from the service line through its colors. The emergency line is represented by red lines and red couplers, while the service line is represented by blue lines and couplers.

Also, they vary in size, making wrong connections to be rare, even though some manufacturers offer the same sizes.

Nonetheless, some people may still confuse these two lines and plug them in the wrong places. If this happens, then the trailer brakes will not work as needed. Besides, it’s hazardous to drive a trailer with cross-connected trailer lines. Apart from that, wrong connections will make air back-feed and leak.

How To Fix It

Unlike other issues that we’ve highlighted above that need the attention of a professional mechanic, you don’t need one in this case.

Simply change the trailer line to its port and the service line as well. Always be careful when connecting the trailer lines. What’s more, the couplers should also match with the service lines.

Damaged Metal Trailer Couplers

Metal trailer couplers or gland hands are essential in the trailer braking system as they help connect the emergency and service lines from the tractor or truck to the trailer.

The couplers are usually fitted with a rubber seal that prevents air from escaping. Additionally, you should couple the right glad hand together to avoid causing issues.

Most metal trailer couplers are colored to avoid using the wrong coupler to connect the towing trailer and truck. Red is usually for the emergency line, while blue is for the service line. In some cases, tags are attached to the lines to identify the service and emergency lines.

If you mix the metal trailer couplers, air will be sent to the service line instead of filling the trailer tank. On the other hand, the pressure will be sent to the trailer tank instead of the service brakes. This is very dangerous as there will be no air to release the trailer spring brakes.

But if the metal trailer couplers are damaged or dirty, they may prevent the smooth flow of air from the tractor to the trailer tank. As a result, this may make air back-feed as well.

How To Fix It

Ensure to correctly attach the metal trailer couplers and lines. If the couplers are damaged and causing air to back-feed or leak it, replace them.


All-in-all, air back-feeding in the trailer emergency supply lines is mainly because of bad valves. These valves normally connect the trailer’s brake system to the tractor’s brake system.

Check and ensure that you’ve connected the emergency supply line and service line properly.

Even though most of these issues can be handled by a DIY enthusiast, most trailer air brake system issues should be tackled by a professional mechanic.