Elecmuso wrote:

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Thanks Swamp. The controller is a Tekonsha Primus with boost modes - I have tried all 3 boost modes and none of them result in me being able to set the gain high enough to lock up the caravan brakes when operated manually.

I agree it suggested extra brake lining clearance.

Interestingly it seems to be a common concern as I have found googling it, for example:
www.jaycoowners.com/forums/f37/trailer-brakes-dont-lock-up-48581.html

-- Edited by Elecmuso on Sunday 7th of August 2022 12:15:33 PM

-- Edited by Elecmuso on Sunday 7th of August 2022 12:16:12 PM

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 The boost mode from memory only works with normal braking mode not the manual overide

The Tekonsha P3 has 4 Boost settings  B is off is when the trailer is lighter than the tow vehicle but B1 can used , B1 is the factory default setting and and if you caravan is about the weight as the tow vehicle B2 and B can be used, B2 is when the caravan is up to 28% heavier than the tow vehicle B1 and 3 can also be used, B3 is suggested when you caravan is up to 40% heavier than the tow vehicle B2 can also be used.

What the Boost does

With the boost off, during a braking event, the power to the brakes starts out at zero and increases with deceleration.

With the boost on level 1, B1, during a braking event, the power automatically starts out at approximately 13% of the power setting and increases with deceleration.

With the boost on level 2, B2, or with the boost on level 3, B3, during a braking event, the power automatically starts out at approximately 25% of the power setting and increases with deceleration.

In a nutshell its like adjusting the brake bias to have you trailer brakes react a little faster, thus not requiring the front brakes  of your tow vehicle do the majority of effort.

INITIAL brake setup important, from Primus manual

Adjusting the Power to the Trailer Brakes (Prior to setting Boost) Once the control has been securely mounted within the -90 to 90 degree range, it is necessary to set the power needed to stop the trailer during a braking event.

1. Connect trailer to tow vehicle.

2. With engine running hold manual full left and set Power Knob to indicate approximately 6.0

3. Drive tow vehicle and trailer on a dry level paved surface at 25 mph and fully apply manual knob.

3 If trailer brakes lock up: q Turn power down using power knob.

3 If braking was not sufficient: q Turn power up using power knob.

4. Repeat Step (3) until power has been set to a point just below wheel lock up or at a sufficient force as to achieve maximum braking power.

5. Using the brake pedal, make a few low speed stops to check the power setting.