Red brake warning light but only when warm

[Thacker]

I have a B7 2.0 tdi and I'm getting the red brake warning light, but not on cold start up only when the car is fully warmed up.

I've replaced the batterry but it is still doing it?

I used to have a B6 S4 and that did it also but only on cold morning / cold start ups? Any ideas?

Is it likely to be the brake fluid reservoir sensor?

Many Thanks

 

[V6 Quatt]

Check the reservoir levels first and top up if required usually if levels slightly low will bring light on