How to Bleed Your Trailer Surge Brakes

You probably know that you need to bleed your hydraulic brakes when you first install them. But you'll also have to perform this procedure any time you open up your hydraulic system for maintenance because it can introduce air into your brake lines, which will keep your brakes from functioning properly. So, if you add a solenoid to your actuator, you'll need to bleed the brakes. Replacing a caliper? Bleed the brakes. Upgrading your lines or fittings? Bleed the brakes. I think you get the point.

You should make it a habit to bleed your brakes about every 12 months or 12,000 miles. You can by all means bleed them more often than that, but I know you have a lot of other things going on, so that timeline is typically enough for most trailer owners.

Can you bleed your trailer brakes by yourself?

This is a question we hear all the time. Manually bleeding your brakes typically requires one person at the actuator and one person at the bleeder valve on the brake. But sometimes you don’t have anyone else to help you, or maybe you’re like me and are too stubborn to ask for help. In that case, there are special tools you can buy to help you do it alone.

However, bleeding the brakes with a friend or neighbor is ultimately the method we recommend. These brake bleeding tools are not always easy to use, so having another person there will speed up the process and help ensure that you don't miss any steps.

Below, I will give you 6 detailed steps to manually bleed your surge brakes with the help of a friend. And if you're still set on doing it alone, just know I told you so.

Fill Master Cylinder with Brake Fluid
Bleed Master Cylinder
Install Bleeder Hose on Valve
Bleed Brake Furthest from Master Cylinder
Repeat Bleeding Process on Remaining Brakes
Fill Reservoir with Brake Fluid

Watch video demonstration of how to bleed your hydraulic brakes.

Do not look directly into the reservoir while pumping the push rod as fluid can squirt up and get in your face. Just don't do it.
Do not allow brake fluid to contact any painted surface as it will damage the finish. To protect your backing plate assemblies from corroding should you get brake fluid on them, we recommend first spraying them down with a rust-resistant spray before you start this process.
Do not reuse brake fluid—it may contain contaminants. Trust me, reusing brake fluid won't save you any money.
Never let the reservoir fall below 1/2 full during the bleeding process—air can get sucked in and you will have to start all over.

Step 1: Fill Master Cylinder with Brake Fluid

To start, remove the master cylinder cap. Fill the master cylinder 3/4 full with DOT3 or DOT4 brake fluid. Make sure to leave the cap off during this process—if you put the cap back on while bleeding the brakes, you can damage the seal inside the cap.

Remove cap from master cylinder. You may have to use a wrench to loosen it if your body-builder days are behind you.

Fill master cylinder 3/4 full with brake fluid.

Step 2: Bleed Master Cylinder

Always begin the bleeding process at the master cylinder. Bleeding the master cylinder will pump brake fluid to the brakes and release any air bubbles from the reservoir.

One of the easiest and most common methods to bleeding brakes is to use your coupler's breakaway lever to engage the brakes and release brake fluid. If you choose this method, make make sure that your breakaway battery is fully charged beforehand. A downside to this method is that it requires you to repeatedly push and reset the lever, subsequently draining your battery, which you'll then have to recharge.

However, there are two other common brake bleeding methods that we've personally tested out, which are proven to work consistently with little to no drawbacks. So below, I'll describe just those two methods for you. There's not just one method that will work for everyone, so it's important to find what works for you. You may even want to test out each method to see which one gives you the quickest and most efficient results.

Method 1:

A lot of actuators offer easy access to the safety release bracket and a hole underneath the actuator where you can insert a screwdriver. Some newer actuators even have the hole on top, which is what you'll see below.

While lifting the bracket, insert your screwdriver into the hole so that it pries the push rod. Use small 1/2" strokes back and forth with the screwdriver until the piston is full of fluid and there are no bubbles visible.

Insert screwdriver or bleeding tool into access hole and push against the push rod using small strokes.

Method 2:

If your actuator doesn’t have easy access for a screwdriver, you can do this method instead. While the trailer is disconnected from the vehicle, hook the trailer safety chains together to form a loop centered below the coupler. Place a sturdy board, like a 2x4, through the safety chain loop. The safety chain loop will help give you the leverage you need to collapse the actuator. It’s best to have a board at least 4’ long so that it extends well above the actuator.

Position the board against the front end of the actuator’s coupler and use it to force the coupler case into the actuator’s outer housing. This process simulates actual braking and will pump brake fluid into the trailer's hydraulic system. Manually pull the coupler case back so it’s fully extended and repeat process until air bubbling stops inside the master cylinder.

Place 2x4 inside safety chain loop and push against coupler to bleed the actuator. In real life, the chains should be tighter than in diagram to provide leverage for the 2x4.

Step 3: Install Bleeder Hose on Valve

Starting with the brake furthest from master cylinder (if you have a tandem-axle-trailer, this will be one of the rear wheels), slightly open the bleeder valve just enough so that brake fluid came come through. Connect one end of a rubber or plastic hose (the hose should be about 5/16” OD, 3/16” ID, and at least 2' long) to the bleeder valve/screw fitting on the wheel cylinder or caliper.

Grab a clear container, it can be glass or plastic, and partially fill it with any brake fluid. Submerge the other end of the hose in the container so that you can observe any air bubbles. Place the container on top of the fender or any place where it’s just above wheel cylinder, otherwise air bubbles could escape back into the system.

Tip: We recommend doing this process with the wheel off so that you have better visibility and access to the valve. However, if you're flexible and don't mind getting your pants dirty, it may be possible for you to do this with the wheel still on.

If you have disc brakes, you may have 2 bleeder screws. We recommend using the top screw to bleed the brakes.

We recommend placing your wrench on the bleed screw first, then putting your hose over it so that you can easily open and close the valve with no spills.

Step 4: Bleed Brake Farthest from Master Cylinder

AKA Grab Your Neighbor and Get to Work

This is where another person's help will be necessary for those of you who are manually bleeding your brakes without the help of a brake bleeding tool.

Method 1:

One person will grab the screwdriver again and this time start making long strokes under the master cylinder. This will push brake fluid all the way to the furthest brake. At the same time, the other person will crack open the bleeder valve (only about a ¼ turn, not a full turn) and close it before the other person releases the push rod.

t's very important to close the valve before releasing the rod because this will prevent air from getting back into the system. Otherwise, you'll have to start this process all over again and waste a whole lot of time and fluid— not to mention, your neighbor probably won't do any more favors for you.

Repeat this process on that same brake until no more bubbles are visible in the container and the fluid runs clear.

Use long strokes under the actuator to bleed the brakes.

Open the bleeder valve and let fluid drain into container.

Close the bleeder valve once no more air is present before the other person lets go of the push rod.

Release the pushrod and bracket.

Method 2:

One person will push the 2x4 against the actuator and pull it back out. This will push brake fluid all the way back to the furthest brake. When that person pushes the coupler case in with the wood, the other person will then crack open the bleeder valve (again, only about a ¼ turn) and close it when you stop seeing air bubbles in the bottle. Again, make sure you close the valve before pulling back the coupler to prevent air from getting back into the system.

Push the 2x4 against the coupler to send brake fluid to the bleeder valve.

Open the bleeder valve and let fluid drain into container.

Close the bleeder valve before the other person pulls back the coupler.

Pull back the coupler to stop the flow of brake fluid.

Step 5: Repeat Bleeding Process on Remaining Brakes

When you see no more air bubbles with the first brake, close and tighten the valve. Repeat step 4 for all the remaining brakes, making sure to go from the brake farthest from the actuator to the brake closest.

To verify this process was done properly, you can activate the actuator again and have someone try to spin one of the wheels. The wheel should not spin freely when activating the actuator is engaged because this activates the brakes, locking up the wheels.

Step 6: Fill Reservoir with Brake Fluid

Once you’ve determined there’s no air left in the system, finish filling the reservoir with brake fluid and re-install the cap. That wasn't so bad, was it?

About the Author

My journey with etrailer started in Customer Service, where I went through months of product training to make sure that I had all of the knowledge I needed to help our neighbors find a solution to any situation. I helped them with technical questions, troubleshooting, product information, and anything else they needed. Since it has always been my passion to write and express myself through words, I made the transition over to the content side of the website so I could combine my product knowledge and passion for writing.

In order to make sure that I am giving you the most accurate, current information, I am constantly doing research and talking with people who are doing what I am writing about every day. I am constantly striving to find out what questions you are asking, and to give you an answer to every one of those questions, plus answers to questions you haven't even asked yet - yes, I'm that good. Plus, I am constantly getting hands-on training with our vendors and asking them all of the hard questions, that way you can have all of the information you need before making a decision.

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Written by: Olivia M.

Don

6/16/2024

I got a new actuator and have it mounted. I have a tandem axle with drum brakes on the front axle. I've tried everything from a wrench to vise grips to bolt extractor and can not get one of the bleeder screws loose. Problem I am having is there is only 1-2" between the screw and the trailer frame. Is there a way te get this off without removing the entire hub or taking off the axle?

Bud M.

6/17/2024

@Don Without being able to see your exact situation it is hard to say what you could try. Depending on how far above the bottom of the frame the bleeder screw is you may be able to raise the trailer from the frame and drop the suspension just enough to have better access to the bleed screw.

Bill

2/1/2024

Hello. I have done full system brake bleeds replacing the entire fluid from the master cylinder and lines on cars before with a 1 person suction system with a fluid bottle placed on the bleed valve. Makes it really simple to completely replace all brake fluid in the system. Considering this, I have a dual axle trailer. Is there some reason i can not do this same process on the trailer while just filling the brake reservoir each time new fluid is sucked through the lines? I was actually surprised you did not have this option outlined in the article as well since it can be done with one person if done carefully. Please let me know. Thank you.

Mike L.

2/1/2024

@Bill There are a couple different ways to bleed a braking system. We recommend the method shown because that's what the hydraulic coupler manufacturer recommends. We try to stay consistent with that. Your method would work just fine.

Bill

2/2/2024

@MikeL Ok, thanks for the confirmation Mike.

Mike L.

2/2/2024

@Bill You're very welcome!

Dirk

8/5/2023

I have a Demco hydraulic surge drum brake coupler. There are two bolts on there that have a decal that says do not adjust. How do I make sure that are tightened at the right amount ? I am guessing too tight and the coupler will not be able to slide when brakes are applied , nor release when I pull away . Advice appreciated. Thank you

Mike L.

8/8/2023

@Dirk Do you have a part number for your coupler? I can get with Demco and get the correct info for you...

Joe M.

10/31/2021

Does this process work for a trailer with drum brakes? I am changing out the old actuator with new one.

Les D.

11/1/2021

@JoeM Yes, this process works for trailers with drum brakes.

Tj B.

1/5/2021

How do i put the hitch back into the trailer. the hitch and the coupling came loose. I have the hitch, but im wondering if i just bleed the hose goes into the master cycllnder. can you explain this for me.

Jon G.

1/11/2021

What exactly do you mean by came loose? Did the coupler part of your actuator completely disconnect? How did that happen?

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