• The front brake lamp switch is in the hydraulic line, under the fuel tank. Remove the fuel tank and you'll find it.

• Your issue could have 2 root causes. I HIGHLY suggest you fix BOTH while you have the master cylinder off and the hydraulic system disassembled. Having the system partially disassembled is to your greatest advantage in doing both repairs.

► It's very obvious... the brake fluid probably needs replacing. DOT4 brake fluid needs complete replacement once a year.

► What is not so obvious is that one or more of your brake lines is probably collapsing internally. DOT4 fluids are so intensely caustic that they totally destroy the unseen inner tubing. (I'm not talking about the steel lines, only the multi-layer flexible hoses.) When this happens, the master cylinder can 'brute force' pressure by the blockage, but then the pressure cannot return to the master cylinder to allow the system to "relax", and so the brake lamp stays ON.

In effect, the brake lamp is simply telling you that there is still pressure on the line. Therefore, the brake lamp switch is not broken at all, but is in fact working perfectly !

I highly suggest you replace ALL flexible lines (front and rear) with what's called "stainless steel" brake lines. These are in fact Teflon hoses with a stainless braid. The Teflon resists the caustic brake fluid, and the steel braid resists expansion under pressure. There are numerous suppliers, but the leader is Spiegler.

https://spieglerusa.com/brakes.html

BTW... the test for internal collapse is to simply blow through the line, once it's removed and washed. If your grandmother couldn't blow through the line, then it's blocked. I'm talking flow at 1 PSI pressure, not what you can do with 100 PSI shop air pressure.

Hope this helps.

PS... See https://www.airheads.org/forum/technical-discussions/1103-brake-hose-failure-causes-cures

found it ok. brake light stuck on. tapped the switch and light went out.
had bike on center stand, both front and rear set and released ok.
bad switch. lines are ok.
now the bad news. they are about $40 with shipping.
looked up on rockauto at cars of the same year but switch is different.
generic is $5.14 for a car.
suggestions for a fair priced one?? Thank you, Dave O

As with the engine oil pressure switch... BMW did not use a special switch. Measure the threads very carefully, then go to one of the older auto parts stores... the ones with books and not simply computers. In the back of the books are conversion and reference tables for doing part research. There will be a section on brake lamp switches and simply look for one with the same thread.

As you suspect, it's the same switch as some German car used during that same era. Then you can order (they probably won't have the part in stock) the generic replacement switch for $4.

I checked the ones for BMW cars of that year, they are different. the pressure switch has 2 prongs. oil sending has 1.
sent email to Airhead Cycles, Tommy Troung. maybe I'll get a reply on Friday.
or Capital Cycle. well if it ends up costing $$ what ever, beats having the bike sitting there.
Dave O

Capital Cycle sells one on e-Bay for $18.95.

You'll have to do a search on e-Bay. For some reason I can't post the link.

If that's not the one, they sell another on their website for $33.95. Still cheaper than the one you found.
Not sure what the difference is. www.capitalcycle.com, https://www.capitalcycle.com/Brake-Switch--Hydraulic-1975-84_p_749.htm

Good luck...