What model and year Triumph?...if its a 60/70s model with Lucas home before dark electrics then its normal. If not I would be looking at what charge is going into the battery when it happens... typically with a full load of lights etc the voltage should not be below 13.5 vdc.The alternator and or reg rec may be causing the fault. Not too sure how to specifically check these and also what output the alternator should give...a specific forum may be your best bet.
Lucas...also known as the prince of darkness... made the worst electrics ever to be put on any vehicle. The alternators they made were barely adequate and failures were common. I had an old Triumph that continually ran the battery down if relying on the lights lol! I used to jam a small piece of wood under the light switch to keep it on and rectifiers were a regular spare part carried....ah the good old days!!...wonder if they still use them?
Joseph Lucas, ‘Prince of Darkness’, alleged inventor of the world’s first self dimming headlights and intermittent windscreen wipers. Who’s light switches usually have three settings – dim, flicker and off. And who apparently still holds the patent for the ‘short-circuit’. The list of Lucas jokes is endless.
Funny stuff right here.I'm sure it brings back memories for ya.:lol4:
Sounds like both your brake light and headlight share a common ground. Check your electrical connections for corrosion including the points at which the electrical system is grounded. The battery or charging system may also be the problem. Connect a voltmeter to the terminals while the engine is running and check the voltage while someone applies the brakes. The change in voltage should be minimal. If you have a significant change then you have a problem.