This long & extensive article covers both disc & drum brakes. There is a lot of information for 2-wheelers, some for sidecars & tugs. There is a complete discussion about brake fluids & bleeding. Much is applicable to any hydraulic brake system. Included is squealing information for motorcycles, especially Airheads, other bikes, Bulletins for K-75, Etc.
Warning: working on brakes is serious business. Read this entire article, perhaps more than once, before you begin work on your brakes. If you do not feel competent, take your bike to a qualified shop.
Broken cable or lever operated brake switch? (NOT the hydraulic switch). Brake pedal bolt not contacting the switch properly? (and it’s not a bent tube at the frame): http://bmwmotorcycletech.info/brakeswitches.htm
Regarding the hydraulic pressure activated brake switches: Those switches, originally 34-31-1-233-959, were replaced with 61-31-1-244-334. If you haven’t access to the BMW part, you can probably substitute the switch used on old VW Beetles. Napa carries that switch as number SL143.
FRONT DRUM BRAKES:
An article written by Duane Ausherman discusses assembly & adjustment of the 1955-1976 front drum brakes, with some applicability to the rear drum brake, & drum brakes after 1976: http://w6rec.com/
In my article that follows, below, I have numerous sections where I get into things Duane did not, regarding the drum brakes.