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Bleading Trailer Surge Brakes

I have a car in the shop that I have a question in another post about and I have a repair to do on the trailer that this car is on too.
A brake line snapped off at the flare nut and I’m replacing a short piece of line.
They are emergency surge brakes with the cable that actuates them if you lose the trailer.
Once I replace that piece of line…how do you bleed these. Or is it just gravity bled at each wheel cylinder.
I didn’t want to go yanking on that cable and find that I just locked the brakes.


You bleed them like regular brakes. Only instead of using a brake pedal you operate whatever applies the brakes.


You’re saving me a second time in mere moments Tester.

So I presume I loosen that little bracket that keeps the tention on the cable and just pump that cable as you would a brake peddle. I think the little bracket just keeps the cable taut inside the hitch so it doesn’t get caught or tangled

I’ve worked on all kinds of brakes, but never these. Always something new to learn!!!


A vacuum pump may be the easier way to bleed them. It’d probably be a pain in the neck trying to bleed them via the actuator.

Where I work we service a lot of trailers like Yosemite’s with surge (hydraulic as opposed to electric) brakes. On these trailers the tongue telescopes into a channel in the frame of the trailer; then it pushes a pushrod that pushes the piston in a single piston master cylinder. All this happens, of course, when the towing vehicle brakes, effectively pushing the tongue into the frame’s channel. Theoretically, you can use a long 6 by 6 to lever the tongue back into the channel & actuate the master cylinder. You join the 2 safety chains together & lay them forward on the floor. The end of the 6 by 6 goes against where the 2 chains connect. Then you stand on the front of the trailer & pull the 6 by 6 back against the tongue, engaging the master cylinder. I have NEVER been able to make the tongue move this way, even with help from 2 other people!

I think Yosemite is right, but can’t make out exactly what he is saying. There is a lever about 3" long sticking up out of the front of the trailer frame, just behind the tongue. It pivots below, where you can’t see it. One end of the “breakaway” cable is attached to the top of the lever, the other end to the towing vehicle. If & when the towing vehicle breaks away from the trailer at speed, the cable yanks the top of the lever forward; the lever pivots & the bottom of the lever goes backward, engaging the master cylinder. Notches on the lever then engage a plate bolted to the top of the trailer frame so the lever can’t release. Now the brakes stay engaged. So, to bleed the brakes, you unbolt the plate & remove it. Put a long vice grips on the lever. Pull lever back & forth to bleed. Re-install plate.

Hey Yosemite! Don’t forget–these drum brakes are not self adjusting. The other week I fixed a leaking brake tube on a trailer like yours. I bled the brakes as per above. No braking action. Boy was I p***ed! Then I adjusted the brakes. That was what it took.

We work on Demco trailers and gravity bleeding is all that is needed when the brake system is opened.