Not sure which brake controller you have the following are the instructions for user adjustment of the Primus IQ.
Adjusting the Power to the Trailer
Brakes (Prior to setting Boost)
Once the control has been securely mounted within the
0 to 70-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 6.0
3. Drive tow vehicle and trailer on a dry level paved
surface at 25 mph and fully apply manual knob.
✓ If trailer brakes lock up:
❑ Turn power down using power knob.
✓ If braking was not sufficient:
❑ 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. Trailer braking is
initiated and terminated via the stoplight switch.
When the brake pedal is released, trailer braking
will cease
I hope this helps