Mu old one used to do it, got nicked before I had chance to look into it deeper. I found no faults with the vacuum reading at the pump, nor any faults in any of the wiring. My next port of call was to take the servo out itself due to a similar and very common problem on A4's and Passat's.
Because the servo sits in the plenum (same design as the 75's) if the drain tubes are blocked the plenum fills with water, as we all know. Water subsequently creeps into the servo itself, or corrodes the servo housing allowing it to separate very slightly, causing a vacuum leak, some customers complained of actual brake failure (in respect of having a very hard pedal), other just the light on the dash.
Another fault they suffer from was an electrical fault; the main starter lead ran right next to some brake system wiring. upon start up (particularly when cold and more amps drawn from the battery) the amperage going down the starter cable would 'activate' the brake wiring as it were and bring on the dash light. Though I can't think of anywhere on a 75 where wiring crosses path in a particularly close vicinity to cause this.
I've not had another one in to do any further checks on but maybe that is of use to someone.
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Customs Inc, MG Rover Enthusiast since '83.
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