Punto brake light problem

About 3 months ago the brake light on my 2001 Punto started to intermittently light up, briefly (1-2 seconds), either when going round lengthy corners, or when braking sharply. I had the car serviced, asked
for the brakes to be looked at - the mechanic found nothing wrong and topped up the brake fluid. However the problem with the light did not go away and may have been worsened by a journey down a three-mile 1-in-4 hill, riding my brakes for most of the way, which resulted in the light appearing again although it went off shortly after resuming driving on the flat. The problem with the light kept recurring, so took it to the garage again, had the whole brake system checked again, and eventually the mechanic found a leaky clutch slave, which he replaced. HOwever, the brake light is still lighting up intermittently (- it appears to happen more frequently in hot weather)- the brake fluid is not low, the brakes have been checked several times, the one leak discovered has been fixed. Has anyone any ideas what could be causing this -are there any other possible leaks? is it possibly a falty sensor and if so how could i identify any problems with the sensors which cause the light to flash on?
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Can't speak for the Punto, but aren't Fiat brake lights operated by a mechanical switch, located inside cabin above the brake pedal, with two fixing nuts, one setting its operating position, the other a lock nut: the nuts can work loose, so the switch operates the brake light when it shouldn't? If that's how yours works, the switch being hidden under the dash, above its mounting bracket, can be hard to get at, plus you can't easily get a spanner on its nuts. On a different model of Fiat I ended up taking the drivers seat out for more room and needing to be less of a contortionist.
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