[Richard Farrant]

Hi Mike,
I recall this used to happen a lot with Ferret scout cars. They had an extension tube on the top of the master cylinder, for convenience of filling, but if filled to the top of the tube they will activate the pressure switch, usually after a drive, suspect that heat of brakes cause the fluid to expand a little. Also filler cap had a vent in it and can get paint or dirt blocking the holes.

regards, Richard