It's a Hall effect and not a switch? That probably needs 3 wires, power, ground and signal. Power is around 4.5V. Signal probably has to be high to signal brakes are off. Disconnect the brakes, and signal is stuck at 0V.

If it doesn't work with a wire between signal and power, then it's like hall effect throttles on many ebikes. They require the high voltage on the signal to be less than power. I believes it's for safety when there is a hall sensor failure in the throttle. As a result, you can't short signal to power to test the throttle input on a controller.

Let's say the above is true for your brake input. You could use two resistors to make a voltage divider, but what I've found to work to test throttles is a diode between power and signal. The + lead of the diode points to signal.