Rear brakes failing on 1150GSA

[bmad]

On two separate occasions now, while driving in rather heavy traffic with plenty stop starts (suburb driving) i have found that the rear brakes have become spongy and then fail completely.

Once the bike has cooled down, all seems fine again. Does not happen in normal commuting conditions  atch:

Any ideas of what could be the problem.  ???

 

[Pistol]

 

[bmad]

 

[Madala]

 

[Welsh]

 

[bmad]