Parking Brake Valve Leaking Air (Won’t Pop or Not Holding)

Even modern systems break down. It happens and it will continue to happen. The key is to know what needs to be fixed and where to get the right parts to make the repair.

Some amount of air leakage is to be expected as it is normal. 1/2 of the parking brake valves returned for suspected leaks are found not to have a problem. To find out if you have an actual leak, test the valve first and use the BR02-39 of the Chassis Workshop Manual under Inspection and Repair.

To learn more about this topic, just continue to read our article. It has the information you want to know about so you can know if you have a valve problem or not. Take a few minutes t see how this information saves you hassles.

Freightliner Parking Brake Valve Leaking Air


There can be different sources for this problem. One may be the diaphragm as it will leak air if it has gone bad. The good news about this problem is that it is rare and may not happen to you.

To make sure it is not the diaphragm, then put a pair of vice grips on the line going to your maxi pot and see if the leak stops. The max pots where the diaphragm is located may only last between 4 and 5 years.

If they are all the same age, then you should replace all of them to stop this problem from taking place. If the leak continues it should be a different part that has gone bad.

Some more advice on this issue- “ take and disconnect the service air lines from all the park brake air cans. These will be the cans with two lines and the line you need to disconnect is closest to the slack adjuster.

Take and remove these lines and then push in your knob and then see if you have air coming from one of the lines and if you do then the air can is bad I have seen the housing between each side of the diaphragm and have an internal crack causing air to back feed into the service side and causing an air leak.” (source)

Yellow Parking Brake Valve Leaking Air


There are 3 common problems with this issue.

1. Leak in Exhaust Port- when you find that the air is not holding in your system, then it is possible that the exhaust port sprung a leak somewhere or was cracked.

The leaking may be in one of the many lines that connect to the exhaust port. If these lines go bad, then air finds those holes, etc., and exits the system. Finding the right hose and replacing it will be one solution.

If you do not find a leak in any of those hoses, then the exhaust port is damaged and needs replacing. Replacing the exhaust port can be tricky as you need to put all those lines in the right position. If you don’t, you will know it soon enough and then you will have to repeat your work till it is right.

2. Relay Valve plumbed wrong- this happens and even professionals make this mistake from time to time. It happens because it is easy to misplace one of the many hoses in the system that connects to the valve port is bad or misplaced.

The line from the dash valve may have been plumbed into the supply circuit of the parking valve. When it is, you have a leaking air problem. The solution would be to adjust your relay valve and then check the hoses. If no leak is found, then you solved the problem.

3. Leaking Brake Chamber- this takes place when the chamber cracks or gets damaged in some way. The first thing to do is reposition the lines and if that does not solve the problem, then you know the source is the diaphragm.

If it isn’t the diaphragm, then you may have to replace the complete brake chamber. That is the easy fix. The repair of the diaphragm is a little more complicated and it goes like this:

To repair the cracked diaphragm, you’ll need to cage the spring first. Make sure to fully insert the spring and then turn it 90 degrees. This ensures that the bolts are engaged in the parking brake.

At this point, you’ll have to take out the clamp that’s holding the diaphragm. The potential to hurt yourself is there if done incorrectly. Carefully remove the clamp and remove the cracked diaphragm.

You can pull the slack adjuster back and use a vise grip to hold the small return spring compressed. This will help you install the new diaphragm easily.

Once you’ve installed the new diaphragm, reinstall the clamp and assemble everything together again. Make sure to take out the cage bolt and reinsert the rubber plug.”

The Air Brake Parking Brake Not Holding


This situation also has a couple of sources for the problem and maybe even three or four. One of the most common issues would be that the brake is out of adjustment. Or if pads are attached, those pads may be past their point of being able to be adjusted.

The first thing to do is try to adjust the brakes and see if that solves the problem. Or if you do have pads, replace the pads and adjust them according to the new pad size.

It is possible that the springs in the maxi cans are broken and need to be replaced before you get your brakes operating normally again. Then there is a slack adjuster on most vehicles with air brakes.

These are on each wheel and you would need the right tools to tighten these adjusters up if they are manually operated. Some vehicles have automatic adjusters and you may have to check the system to see if it has failed.

Some RVs have what is called air assist brakes. If these are not helping the hydraulic system to hold your RV in place, then you may have a problem with one of the cylinders or both of them.

Have them checked out as one cylinder will help the regular brakes and the other helps the parking brake.

The Parking Brake Valve Won't Pop


Bendix sent out a news release in 2019 about this issue. Here is what one part of that news release says about the yellow parking brake control knob:

The parking brake control valve – the one with the yellow button – has no Federal Motor Carrier Safety Administration (FMCSA) regulations specifying the pressure at which it must automatically trip to apply the vehicle parking brakes. Parking brake “pop pressure” isn’t addressed in Federal Motor Carrier Safety Regulations (FMCSR) for in-use vehicles, the Federal Motor Vehicle Safety Standard (FMVSS) for newly manufactured vehicles, or the out-of-service criteria checked during inspections such as roadside checks by the Commercial Vehicle Safety Alliance (CVSA); although FMVSS 121 does require a single control for the park valves on a tractor-trailer and all the vehicles in a combination train.

That said, the FMCSA Commercial Driver’s License Manual includes a note in the air brake section that says the parking brake valve should close, or “pop out,” at approximately 40 psi – a specification that isn’t supported by regulation, and which can cause some confusion.” (source)

As you can see by that news release, the yellow button will only pop out if the right air pressure is in the system. Depending on who made the air brake system, that 40 psi level may not be universal for all systems. Here is what Bendix has to say about checking the air pressure:

- Install an accurate pressure test gauge in the secondary service reservoir

- Release the parking brake by pushing the yellow button

- Charge the system to air governor cutout

- Turn off the engine

- Open the manual drain valve (petcock) on the primary service reservoir to completely empty the tank

- Open the secondary reservoir’s manual drain valve and use the test gauge to create a bleed rate of 20-50 psi per minute

- Watch the test gauge and note the pressure at which the yellow valve pops out.” (Same source)

Parking Brake Valve Cost


As you already know, the cost you pay will depend on the outlet you like to shop at. Each outlet will have its own price for this valve and the cost will depend if you just want the Yellow knob or if you want both the red and yellow knobs on the same valve.

The website Anything Truck has a wide assortment of valves. There are 13 pages to that selection and you will pay anywhere from $18 to over $330. The most expensive valves are the two-button options, at least on page one of that selection.

Amazon also has a few options to choose from. The most expensive would be the Bendix model. It is priced at over $230 but it is a better deal than the previously mentioned website.

You can check directly with Bendix to see if they sell to the public. We can’t guarantee that you will pay less but one thing is for sure, you know you are getting genuine Bendix valves.

If you go to a repair shop, then expect to pay between $218 and $260 for the valve. Then an additional $110 to $140 in labor to install it. You may find better deals near you and it will pay to do a local search.

How To Replace The Parking Brake Valve


The process can be quite long as there are many steps involved. The first step is to remove all plastic covers or trim pieces with a special tool. When those pieces are off, remove the screws that were hidden underneath. There will be a few of these screws so make sure you have the time to do the work.

Once the screws are out, you need to lift up and turn counterclockwise to remove the yellow and red knobs. Then remove the plastic piece underneath those knobs where you will find more screws to remove.

Leave the center screws alone and only remove the side ones. After removing the 4 side screws, the valve should come right out. But leave it in place for now as you need to remove the top plastic access panel to see the air hoses under the dash.

Next, remove the plastic panel next to the valve, This panel should have rocker switches on it. Mark the air hoses so you know their position and avoid being confused.

Once you have room to work, take hold of the individual hoses, and push in, turn, and pull out to remove them from their connection. Each hose should come off easily and once off, the old valve can be removed without any trouble.

Putting the new valve in place is only a matter of placing the hoses in their correct position on the new valve and returning the valve to its original position.

Then it is a matter of reversing your steps and replacing all the parts you removed to gain access to the hoses. It is a time-consuming task and you can see all the instructions, a video, and other information below.

Some Additional Words

Air brake systems are very good braking systems to have on your vehicle. They work well and make sure your vehicle does not go any place. But when they are not working as they should, you do have to do some repair work to them.

The above information should help you get those repairs done quickly and easily. Of course, you can leave the work to an air brake mechanic if you are not up to the task.

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