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Yep and that tube tees into 1 of the metal brake lines.
When you step on the brakes the pressure is transferred to the controller that changes it to electrical and then send that to the electric trailer brakes.
If you follow the electrical side out of the controller a wire should go to an adjustable slide resister then to the trailer.
This adjustable resister is for when the trailer is empty / loaded as the braking power need to be changed or the wheels can lock up when empty.
This type of controller went away when ABS came out as they did not play nice together.
The new types, there are 2 different ones, all the adjusting is done at the controller and work really good as I have one in each of the 2 tow rigs I have.
Dont go for the cheaper Timed Apply as you cant really got then to work right, or I could not on the 1 I had for a short time.
The other type uses a pendulum and as you slow down it moves to apply power to the trailer, hard you stop the more it moves the more power out. and as said you can adjust the output right at the controller.
Dave ----
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