Help with Brother BAS 415

First check your bobbin tension. I do the "drop" test. While holding the thread the bobbin case should drop slowly. If it does not drop at all, loosen it. If it drops too fast, tighten it. Do you know how to loosen and tighten bobbin tension by turning the little screw? Now for the upper thread, do you have a tension gauge? If not I would get one. I set my upper thread tension to about 125 by turning the big tension knob. You should not have to change the pre-tension knob very often, only as a last resort. Sew out an H and check the back. You should see about 1/3 bobbin thread showing. Do that for each needle. If you are getting frequent thread breaks it might not be the tension. Narrow it down. Does it happen on all needles? If so it may be due to a nick in the pressor foot, rotary hook, etc. If it's just one or 2 needles try changing your needle. Sometimes the design itself is to fault, too dense or other defects.