1983 110 brake warning light on. Why though?

I have a well-used '83 110, and just recently the brake warning light has been coming on and remaining lit. Brake fluid thinks I, but no, in fact it's up to the MAX line. Passed its MOT 8 weeks ago so the brakes themselves are OK I think.

Looking at the brake fluid reservoir, there doesn't appear to be a sensor, so what is the warning light telling me? The Haynes manual only seems to refer to the Defender 110s, not mine, which is just before the Defender badge came into use.

The brakes seem to be working OK but naturally I'm a bit wary of driving when there's a big scary warning in my face, telling me they aren't!

Hand brake warning light maybe?

Hand brake not releasing fully? Or dodgy connection at handbrake lever?

Haven't you got two wires attaching to the top of the brake fluid reservoir

Brake imbalance warning,could be due to a leek in one of the brake lines or faulty switch,on chassis near footwell.

As per tottot, when mine came on it was due to a pressure imbalance being detected. One of my cylinders was leaking slightly. To get it to light, I had to hit the brakes hard.

No there's no wires. Just the reservoir. No electrical connections anywhere.

The light comes on as soon as I turn the ignition and stays on.

Update: This problem was fixed by the repairers after the vehicle got nicked in January and had to have a new steering column fitted. I still don't know what was causing it but I suspect the sensor switch rather than the brakes themselves. Anyway, they sorted it as part of the insurance job.