If I recall correctly, years ago somebody had Servo ABS faults which eventually were traced to the rear brake light cluster itself (don't ask me why). I know the 2004-2007 bikes had badly sealed rear lights which let in mud and water from the rear wheel causing corrosion to the lamp holder inside the unit, this somehow upset the Servo ABS maybe through a short circuit or backfeed perhaps ?
I know my old 2004 GS had a rotten internal brake light lamp holder when I checked it. Its no wonder the 2007 onward bikes went to a sealed LED brake light design. FWIW I hated the ServoABS on my 2004 bike and was glad when I upgraded to a 2008 GSA.
Either way, why not open up the rear brake light cluster and see what state it is in, it couldn't hurt matters and you might get lucky ?