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Your Boat Trailer Brakes!

Technician: "Sir your trailer brakes lines are full of rust and the reservoir is empty."

Customer: "My boat trailer has brakes??"

This above conversation seems to happen more often than not in the shop. When asked to look over a trailer or inspect a boat package for a pre-purchase sale more times then not we discover the trailer brakes are contaminated and not working properly. Lots of boat owners are not even aware their trailer is equipped with brakes which is half the problem. Not having your trailer brakes working can lead to a major accident especially in the case of a heavier boat and smaller tow vehicle. Here we will break down how your boat trailers brakes actually work so you get a better understanding of them and the required maintenance.

Surge Brakes

For the case of this blog, we will be talking about surge brakes as they are the most common on the recreational boat trailers. The other brake option is electric brakes which would be mostly found on holiday trailers or work trailers. Surge brakes or sometimes referred to as hydraulic brakes work when the tow vehicle engages the brakes, the actuator on the trailer slide backward pushing fluid down the brakes lines to the brakes. The trailer actuator actually moves within the coupler frame of the trailer allowing it to move fluid down the lines when required. When the tow vehicle starts moving forward again a pulling motion is causing the actuator to slide forward within the frame and release the pressure in the brakes.

Brake Fluid

Most trailer manufacturers call for a DOT III or greater to be used in the reservoir. The reservoir is located right on the top of the actuator and can be accessed with a flat head screwdriver. Take the cap off and have a look at the level, the fluid level should be right up to the bottom of the cap. If not add some more fluid to top up. Your trailer brake system should be a completely closed system meaning in a perfect world there should be no need to top up the reservoir. Being on low on fluid could mean there is a leak in the system and is something you will want to monitor.

Brakes Lines

Your brakes lines can be made up of steel or rubber. More often then not we will see steel lines running from the actuator back to the first axle and then possibly a rubber line going on from there. As you can see in the video above the brake fluid is pushed down the lines from the actuator to the brakes. Any hole or damage to these lines allows the fluid to leak out and lets air enters the system, and in a lot of cases water because of constant loading and unloading of the boats. If you suspect a damaged line it is best to have replaced as soon as possible to delay any further damage that may occur.


The brakes can consist of either drum brakes or disk brakes, either or are acceptable. They are much like the brakes on your vehicle and should be inspected regularity for any worn parts or damage. If planning a big holiday with the boat where you will be towing it a great distance it is advisable to have the complete trailer checked over before heading out. Have your shop look over things like the brake pads and lines, wheel bearing, tires, etc.

Reverse Lock Out

The dreaded reversing issue that can accompany many surge brake trailers. If you’ve had this happen to you, you know how frustrating it can be. This typically happens when trying to back up a driveway or anything with a slight incline the brakes will lock the tires up making it impossible to back the trailer up. This happens when reversing uphill or on a flat surface because as we know the actuator is moving backward when you are pushing the trailer in reverse. It is not such an issue when reversing down into a boat launch because the weight of the boat keeps the actuator forward in the frame. Some manufacturers have a built-in solenoid in the actuator that works off the reverse lights; when the reverse lights go on it is suppose to allow the brake fluid to bypass and not engage the brakes. These can be identified by a 5 wire trailer harness. The blue wire is the lead for the solenoid and if the trailer will lock up on you with the lights hooked up this could be the issue. Other actuators simply have a reverse pin lockout where you physically have to install the pin in an area within the trailer frame and it prevents the actuator from moving backward within the frame. This pin must be removed before towing though or else the trailer brakes will not engage.

When working properly your trailer brake system will be of great aid when doing lots of towing. Be sure to have your trailer inspected regularly and to have the brake fluid level checked at the same time.

Have any questions or comments? We would be happy to hear from you in the comments below!

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