Have you tried setting the stitch length to zero?
For my Janome, I got a special bobbin case that is for FMQ--the tension is adjusted properly for that, so I just change the bobbin case when I'm ready to FMQ. I don't know if Brother has a FMQ bobbin case, but I think I would invest in a separate bobbin case, set for FMQ, rather than constantly changing the bobbin tension. Good luck.