Wouldn't the idiot light come on if problem with ABS before it's needed.
No. The ABS module is valved off during normal braking.. That's why they flush the system with the digital tech.
A good point was make to clean the caliper piston of any crud. I had a front lever on a bike that went right to the grip, changed out the fluid and bleed the calipers, No change so I, mentioned it to a HD tech I know and he advised to clean the calipers. I doubted his advise but tried it anyway, son of a gun, front lever became solid and bakes worked great.
So dealer says everything looks good front break acting normal , including calipers and rotor, but rear fluid needs flushed. So they are going to flush front and rear. They're thinking maybe other dealer didn't stick to protocol for flushing abs. So I'll see how it does when I get it back. Hmm
Is 5600 miles a typo? If not, the seals may be getting bad from non-use. Hydraulics (like most mechanical systems) like to be exercised regularly. Also, a fluid change at 600 miles means less than the time elapsed since the last change. Hydroscopic fluids scavenge any moisture that infiltates porous membranes like the rubber gaskets found throught the brake system.
Miles is right. I don't know how many times brakes were flushed but worse case, it was the 5000 mile service. I put 600 miles on it and it was fine, not sure it was their fault. It was stored at dealer in winter. Rotar and calipers fine. Little weird for me, had bikes for many years, new to ABS LInk though.