I did what I should have initially done - researched the Brake Buddy. I now understand that you have the Vantage Select model.
It sounds as though the sensitivity setting to activate either proportional control or full control is quite variable. Perhaps the pendulum is damaged or defective? I would think that regardless of the mode, this sensitivity adjustment is required to determine the exact point your system should start braking. It should consistently sense the amount of de-acceleration of your coach. I'm wondering what could affect the sensing process? I would think that if the BB slides around on your floor because of too much play in the arm connection to your brake pedal, this could result in variability, especially when stopping. The brake unit will slide forward on the rug until there is enough force on the arm from the brake pedal to stop it. THEN, the pendulum will come into play & recognize that the MH is stopping. I have bunjee-cord attached my Blue Ox handle to the driver's seat to reduce some of the bounce & slide motion.
I can't figure how the pressure setting affects the proportional control. I'd thought the pressure should be full, but modulated by the BB, like your ABS brakes.
I just know that I set my pressure setting on the Blue Ox to about 50psi, & I keep the sensitivity rather low (hi?) so that I minimize the number of operations. I try to carefully adjust the footpedal space to prevent false brake light signals (which certainly runs down the battery) & really scares the beJesus out of me when the red brake light comes on in the MH cab.... and I tie the brake system to my driver's seat so that it can't wander forward or sideways, either in a stopping condition or merely bumping along the highway.
Sorry for the long post -