Well nuts.
I'm in the process of getting an old but worthy 1985 BMW K100RS back on the road.

When I first got the bike it fired up, but the red triangle 'brake warning' light came on in the speedo cluster. The front brake would NOT activate the brake light at the tail-light. Rear brake did activate the tail-light.

I knew the brakes worked (I pumped/bled fresh fluid through both the front and rear, and they work strong). Then, after about 10 miles of testing the bike around town, the warning light turned off. And the front brake would activate the tail-light. COOL!

In the course of restoring and shining things up, I removed the speedo cluster, and then put it back. NO! Brake warning light is back on, and now the front brake will NOT turn on the tail-light.

I've checked the wiring to the little switch on the front brake lever, and pulled off the little connector. Shorted the wiring on that connector to see if that would turn on the tail-light: Nope.

Hopefully, somebody has had a similar problem. What should I do next? This may be a grounding problem, maybe? Maybe a problem in the speedo cluster?

Anyone else battled with a Brake Warning Light problem? Help HELP! Thanks in advance!

Have you checked the connection at the bulb monitoring unit?
Do you have continuity between the front break switch and the bulb monitoring unit?
Both switches (front and rear) are feeding their signal to the bulb monitoring unit. The unit powers the stop light.

Whoa! Great info and THANKS! Where is the Bulb Monitoring Unit? In that box of relays and junk underneath and behind the gas tank?

Update:
Checked all the connections at the Bulb Monitoring Unit. Those seemed fine. Then read elsewhere on this Forum about the brown ground connectors, all bolted to the frame under the gas tank... so I cleaned all those. Pulled the rear bodywork off to get to the tail-light (yeah, there IS an easier way, I know), and that wiring was good. dropped the tail-light housing which broke the parking light bulb element (it was old), so I replaced that, and FINALLY:

I tried shorting the connection under the gas tank - the connector that runs the little wires up to the switch at the brake lever - SUCCESS. So all along, it was a flakey switch at the hand brake lever. New switch and wire was $20 at my local BMW dealer, in stock, not bad.

Lessons learned:
Twenty-five year old switches can test okay on a voltmeter, but not hold enough current to actually work.
Cleaning the ground connections didn't hurt.
I cleaned all the plug-in relays in the plastic box-o'-relays near the battery. Last time that was done? Who knows?
The red brake triangle warning on the speedo cluster signals if there is a problem with the brake lights. IT ALSO will stay on if one of the parking lights (front or rear) is burned out. Even if the brakes test okay.

THANK YOU to the Crazy Frog person for help with this, and the cool diagrams. Never would have guessed that the brake lights were actually activated by a plug-in logic module.

Great job! Give yourself a big tap on the shoulder.
Thanks to remind us about the warning light working on the tail/parking light too. I forgot about it.
The problem with the brake switches (plunger type) is that the plunger is made of plastic and it wears off to the point that 1st it's no longer waterproof and second the notch dug up by the wear prevents the switch to spring out correctly.
The switch is activated when the plunger comes out. By the way the same type of switch is used for the clutch and the back break on the early K100.

Bert

The lack of current consumed by a meter is one reason why a test light can actually be superior in testing continuity across a switch or contact, the test light bulb need current to light and if the switch is high resistance it reads ok on the meter voltage range but cannot pass enough current to light the bulb.
Regards,
K-BIKE

Thank you for this thread.
I have recently purchased an 87 K100RS with the brake light intermittently on.
I don't have the owners manual and so didn't have a clue as to what the light was trying to tell me.
I'll check the switch and bulb control unit.
Thanks again.

I've had similar problem. Riding along and at about 5 - 10 min intervals, the warning light would light up, flickering at first and then on solid. It would clear by dabbing the brakes. Didn't have a clue. Finished by removing all the lamps, cleaning the lampholders and sockets and then a serve of DeoxIT and...Presto!! all was good.