Stop warning light on dash when after braking the pedal is released

the large STOP warning light comes on after braking and the pedal is released.This does not happen every time I brake.I've spoken to a vw mechanic and he along with a few other mechanics suggested it could be the expansion bottle level sensor [ the sensor and bottle are all one].I changed this but no change so next I changed the brake light switch and still no change.I've tried plugging into a diagnosic tool but no error codes come up.If I cure the problem I'll post answer .thanks Keefy.