How to bleed boat trailer brakes

Bleeding boat trailer brake systems can seem intimidating to many anglers. However, it’s part of routine maintenance for larger boat trailers. The larger boats get, the heavier they weigh, and can easily exceed the stopping power of your tow vehicle. If your boat exceeds 19 feet in length, trailer brakes are recommended. With that said, many manufactures are putting brakes on trailers as standard features. To keep these brakes in working order, you’ll need to perform inspections and bleeding when necessary.

I had been noticing that my trailer brakes were not as effective as when the trailer was new. They were not working as well as I felt they should be. I also noticed an increased amount of rust building up on my rotors. This is evidence that they were not working at all. Since I was putting 4 new tires on the trailer, I decided this would be the perfect time to diagnose and fix the braking system on the trailer.

I have a 1998 518SVS Comanche with a tandem axle Ranger Trail Trailer, which uses a surge braking mechanism built into the tongue of the trailer. This is a very common braking system for boat trailers, and most boat trailer manufacturers are using this system. It is common among anglers that are not knowledgeable or fear attempting to work on trailer breaks, to disconnect or bypass them. The fact is, if you have the knowledge to do this, you have the knowledge to bleed them. I am a firm believer in brakes on trailers, and would only bypass them in emergency situations to get off the road, to the nearest place to repair them.

I have outlined the process I went through as I purged and bled my trailer brake system. Between the photos and this article, I hope you find the confidence needed to inspect and take care of problems associated with your trailer brakes. It could save your life, or that of someone else.

I started out by jacking the axle up and removing the wheels. Placing jack stands under the axle for safety. Next, I removed the cap on the brake master cylinder reservoir, located on the tongue of the trailer. In my system, I found a nasty coagulated mess, rusty mixture.

For those that don’t already know, brake fluid is a Glycol-Ether based product (DOT 3, 4, and 5.1). Brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. This moisture can wreck havoc on a brake system. The moister that has been absorbed in the fluid, will rust the steel brake lines, and decrease the brake fluid’s effectiveness by lowering the boiling point, brakes get hot when used.

Since my fluid obviously had a lot of water in it, I used a turkey baster to remove the fluid from the master cylinder reservoir. You can buy a turkey baster at a Flea Market for about $1, and you’ll find all kinds of things that it’s handy for besides basting turkeys.

Using the baster I was able to suck all the fluid out, and cleaned the left over residue the best I could with a rag. Next, I took a small Acid Brush, normally used for applying flux paste when soldering, and scrubbed the master cylinder. Once the big chunks were out, I poured clean DOT 3 brake fluid in the reservoir and continued using the brush to finish cleaning. I used the fluid itself to “clean” the inside of the reservoir, and break loose any sludge that was still in there. Finally, once again using the turkey baster to remove the fluid I just used to clean with.

Once it was completely clean, I emptied the reservoir and filled it up to the top with new fluid.

A word of caution here, brake fluid will eat paint. Immediately remove spilled brake fluid from the tongue of your trailer with spray parts cleaner. A bunch of old rags will also come in handy.

Once I was satisfied with the reservoir, I moved to the rear to deal with the contaminated fluid still in the lines and calipers. Using a 3/8″ wrench I opened the rear bleeder valve on each brake caliper. On a clean system, you can gravity bleed the fluid from your brakes. I wasn’t surprised when I couldn’t. Due to the high level of contamination, the fluid wouldn’t freely flow. I closed one bleeder off tight, and I pulled out the heavy artillery.

I connected a brake flushing machine used for automobiles to the bleeder valve on one side, just using the suction function of the machine I turned the pump on and let it start pulling a deep vacuum on the system.

Returning to the tongue area to keep an eye on the fluid level, it took a long time for it to start dropping, which indicates the flushing machine is pulling fluid through the line. I realized the lines were clogged. Letting the machine continue to pull on the system, it took a while to overcome the blockage and get a deep enough vacuum to break the junk that had built up in the line loose.

As the master cylinder fluid level dropped, I added fluid and just let the machine do the work.

A note of caution here, the reservoir on my trailer is extremely small and probably only holds 3-4 ounces of fluid; it is very important that the fluid level does not reach the bottom or you will allow air to enter the system.

Switching to the other side I repeated the process.

I ran a total of one bottle of fluid through the system until I was sure it was nothing but clean new fluid coming out of the bleeders. Once I saw clean new fluid at the bleeder valve, I closed the bleeders off, reinstalled the rubber caps on the bleeders, and checked my reservoir level one more time to make sure it was still full.

Cleaned everything up! And ready to hit the road to try them out!

Now I realize most people don’t have access to a brake flushing machine, but in my opinion, the DIY (Do It Yourself) mechanic can purchase or rent (Advance Auto, Oriellys, Autozone) a hand held brake bleeding tool that will also work well. With the hand held tool, you’ll need someone to watch the master cylinder reservoir for you to make sure the fluid level is topped off as needed. A hand held bleeder is a great investment as it will assist with the maintenance on your tow vehicles brakes.

I hope this has helped someone else to maintain their Hydraulic Trailer Brake system.

Rick Olson

If you are not skilled in performing the following procedures, have a qualified service shop do the job.  Check that all hydraulic fittings are secure. Read and understand all instructions before starting. Two people are required for manual bleeding.

1. Remove the master cylinder reservoir plug and fill the reservoir with brake fluid. Use either DOT 3 or DOT 4 automotive brake fluid. Follow instructions on brake fluid container. Avoid shaking brake fluid container and pour liquid slowly to minimize air entrapment. Let fluid in reservoir stand until completely free of air bubbles.
2. IMPORTANT: Before bleeding brake lines, bleed the actuator master cylinder. Insert a screw driver through the hole in the bottom of the inner member and use short strokes to pry on push rod (while holding safety release bracket up) until no air bubbles are seen coming from small orifice hole in the bottom of the master cylinder reservoir.
3. Start bleeding procedure on the brake furthest from the master cylinder.
4. At the brake assembly, connect a transparent bleeder hose to bleed screw fitting on wheel cylinder and submerge free end into a container partially filled with brake fluid. Do not reuse this fluid.
5. The first person strokes the pushrod slowly while holding safety release bracket up. The second person opens the bleed screw fitting. He then closes the bleed screw fitting BEFORE the first person SLOWLY releases the pushrod. Repeat this procedure until the fluid expelled from the bleeder hose is free of air bubbles. Remember to always tighten the bleeder screw before releasing pushrod. During this process, the master cylinder reservoir fluid level must be maintained at no less than 1/2 full.
6. Repeat steps 4 and 5 for the other brake and the brakes on the front axle, if equipped with tandem brake axles.
7. If the installation is tandem axle with brakes on both axles, repeat bleeding procedure on rear axle brakes for the second time to assure purging of all air in the system.
8. As a final check after bleeding is completed, stroke pushrod and check to be sure brake system is pressurized by attempting to rotate a tire.
9. Push up on the safety release bracket to ensure that pushrod is in the released position.
10. After bleeding has been completed, re-check fluid level in master cylinder. Fill the master cylinder reservoir to the indicator on reservoir plug. Do not overfill.

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