Loosing temp gauge when pressing brakes

[jes*b]

Fired up the Westy today . Passed MOT . So all good there. However seem to have developed a fault whilst laid up .... When I press the brakes ( especially noticeable on long braking stretches down hill ) the temp gauge falls , eventually to totally cold. Come back up when off brakes. Not checked if anything else effected whist braking but all lights etc tested for MOT obviously . Ideas ? Is there an obvious link

[tonytech]

Looking at the wiring spec in Bentley, I think the coolant sensor and the rear light cluster share a comon earth in the engine bay.
Do any of the other lights (side or reversing) produce a similar effect?

T

[MidLifeCrisis]

I'd also try pulling out the 42/43 relay and see what effect that has - that would narrow down the search for the culprit a bit?