How to Bleed Hydraulic Brakes on a Trailer

by Don Bowman

Hydraulic surge brakes on a trailer need no wiring or hydraulic lines from the towing vehicle to operate. These units are completely self-contained with their own separate master cylinder. They operate strictly on differential inertia between the tow vehicle and the trailer. When the tow vehicle brakes, the front of the trailer moves forward because of momentum. As it moves forward, it depresses the pushrod activating the trailer master cylinder and applies the brakes. When the rod is released or vehicle is no longer braking, the spring pressure within the brakes forces the hydraulic fluid back into the master cylinder. They are also equipped with a breakaway system mandated by law to apply the brakes on the trailer should it come loose from the tow vehicle. This system is a chain and cable connected from the trailer to the tow vehicle. If the trailer becomes detached, the chain will hold the trailer behind the tow vehicle and the cable will pull on the emergency braking rod on the trailer master cylinder applying the trailer brakes.

Loosen the brake bleeder screw on the brake farthest away from the master cylinder. Attach the 5/16 tube to the brake bleeder screw and put the clear bottle, filled half way with brake fluid up on the fender and insert the hose into the bottle. The bottle must be just above the wheel cylinder in the brake to bleed properly.

Remove the master cylinder cover and fill the master with brake fluid. Replace the cap. Insert the common screwdriver into the ½-inch hole in the coupler below the nut on the pushrod, which extends outward from the master cylinder in the front of the brake coupler housing.

Push the screwdriver in an in-and-out motion to a depth of about ½-inch to force hydraulic fluid to the brakes and eliminating the air in the system. Do this for about 10 strokes.

Tighten the brake bleeder screw and remove the hose and the bottle of brake fluid on the fender and move to the opposite side and do this the same way.

Before the next side is bled check the master cylinder for fluid level and correct as necessary. When finished with the second brake, as a last check, fill the master cylinder.

Items you will need

About the Author

Don Bowman has been writing for various websites and several online magazines since 2008. He has owned an auto service facility since 1982 and has over 45 years of technical experience as a master ASE tech. Bowman has a business degree from Pennsylvania State University and was an officer in the U.S. Army (aircraft maintenance officer, pilot, six Air Medal awards, two tours Vietnam).