I've experienced this issue msyelf...
I ignored it for 6yrs and just focused on brake pads and rotor replacement.
The latest problem along with the continuous ABS light On issue was the master cylinder started to run dry on fluid. Now air is in the lines. Now I need new master cylinder.
Bottom line, the master cylinder has the ABS Modulator Assy attached to it...so bench bleeding was fun considering that I discovered that the RL port would not pump the fluid in or out. Great...the ABS Modulator Port for the MC is clogged or blocked. The other three ports are fine...was able to bleed them with no problem. The culprit for the ABS light on dash may be the ABS Valve Solenoid...which measured at 3.5 Ohms on the rear...no clicking action when 12VDC applied across the pin terminals...that's a $63 replacement.
Still working on resolving this...and I will. Will update you once completed.