A80 Brake fluid warning

Hi,
Up till today, brake fluid warning lamp flashed for about 10 seconds on startup (self test), then stayed out.
As of this evening, it flashes all of the time
Fluid level is fine, and shorting out the wiring at the reservoir doesn't change the symptoms, so the res level switch is OK.
Anyone got any idea what else is in the circuit ?
Cheers
DA
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