How to Check and Care for Boat Trailer Surge Brakes

Park on level ground with the trailer hitched, the tow vehicle in park and the wheels chocked. Hitched is best: the coupler sits in its normal, extended position and you can test the lockout.

Look at the sliding part of the coupler. It should be free of heavy rust, cracks or bent parts. If there is a rubber boot or bellows, check it for tears. Look for wet spots or drips around the master cylinder and where the brake line leaves the coupler. Make sure the breakaway cable or chain (if fitted) is in good shape and its clip works.

Wipe the area around the reservoir cap clean first so dirt can't fall in. Open the cap (on most actuators it's on top of the coupler housing; your manual shows where). The fluid should be near the full mark your manual shows, typically close to the top but not filled to the brim.

Look at the fluid too. It should be clear and light colored. Dark, milky, cloudy fluid, or rust and grit in the reservoir, means water got in and the system needs a flush by a shop. A BoatUS trailer repair tech suggests taking a close look inside the reservoir for rust every time you check.

Add only the brake fluid type marked on the cap or listed in your manual, from a fresh, sealed bottle. Don't mix types and don't substitute. Seals inside the system are made for one fluid. Close the cap tightly and wipe any drips off the paint.

The level drops slowly as shoes or pads wear. A sudden drop means a leak. Find it before you tow.

Follow the lines from the coupler back to each wheel. Look for rust scale, kinks, crushed spots, wet spots and rubber hoses that are cracked or swollen. Gently move the lines where you can; corroded steel lines can look fine and still be thin. Anything leaking or badly rusted is a shop job before the next tow.

Electric: with a helper, put the tow vehicle in reverse with the parking brake set. Listen at the coupler for the click of the solenoid, and check that the backup lights on the vehicle come on (the solenoid usually shares that circuit). No click usually points to the plug, a broken wire, a bad ground or a failed solenoid. Manual: practice setting and releasing it. Confirm it moves freely and that you know where "locked out" and "normal" are.

Then back up a few feet on level ground. The trailer should roll back without the wheels locking.

Jack under the frame (Dexter recommends the frame, not the axle tube or springs), set a jack stand and spin the wheel by hand. It should turn freely with maybe a light brush of the brake. A wheel that drags hard, grinds or won't turn has a sticking brake. Also check for fluid on the inside of the tire or wheel, which means a leaking wheel cylinder or caliper. Grease there is a problem too: Dexter warns that a hub seal that leaks grease onto the brake linings can leave the brakes useless, which is why it says to fit a new grease seal whenever the hub comes off for its bearing inspection.

On disc brakes you can usually see pad thickness through the caliper and the rotor surface: deep grooves, heavy rust pitting or a lip on the rotor edge mean it's time for service.

As soon as you can after pulling the boat out, rinse the wheels, brakes and hubs with fresh water. If your trailer has a brake flush kit (a hose fitting on the fender or frame that sprays inside the drums), hook up a garden hose and let it run for several minutes. BoatUS suggests moving the trailer forward a few feet while flushing so the water reaches all of each drum. No flush kit? Aim the hose into the back of each wheel and drum.

Many owners stop at a car wash with a hand sprayer on the way home. Just do it the same day.

After a long drive, give the brakes and hubs some time to cool before you back down the ramp when you can. Hot parts dunked in cold water suck water in as they cool, past seals and into drums and bearings. See the wheel bearing guide for more.

The sliding part of the actuator needs to move freely, or the brakes can stay on or not work at all. Discover Boating notes the actuator may need occasional lubrication. Use only the lubricant and points listed in your actuator's manual. Grease the hitch ball lightly too. Many actuators also have a shock absorber (damper) inside that keeps the coupler from banging in and out; if the trailer jerks or clunks every time you slow down, ask a trailer shop to check it. Rebuild kits exist for many actuators.

• Brake early and smoothly. Surge brakes respond to how hard the trailer pushes.
• On long downhill grades, anything that keeps the trailer pushing on the coupler (riding the brakes, or slowing hard with the engine) can keep the trailer brakes applied the whole way down and overheat them. Follow your trailer and actuator maker's advice for mountain driving.
• Don't use a weight-distributing hitch unless your actuator maker approves it. It can stop the coupler from sliding and the brakes from working.
• After backing up, shift into drive and roll forward a little before setting off so the coupler fully extends and the lockout releases.

Have a trailer shop or brake shop pull the drums or check the calipers every few seasons, or every season if you launch in salt. Mercury and Discover Boating both recommend this, and you can combine it with a wheel bearing repack. Ask them to check the shoes or pads, wheel cylinders or calipers, hardware, lines and the master cylinder, and to flush the fluid if it's dirty. Log the date in BoatMend.