If the brake fluid is low, where did it go? There is either a leak that must be addressed, or the fluid will go lower as the front disc brake pads wear. That's related to how they constantly self-adjust. In that case the fluid level will go back up when new brake pads are installed. That's why we never top off the brake fluid during other routine service. If you do check or add brake fluid, be absolutely certain you do not get any hint of grease or other petroleum product in there.
Once the yellow ABS warning light has turned on while driving, there will be at least one diagnostic fault code stored in the ABS Computer. Having that code read is the place to start. It will identify the circuit or system that needs further diagnosis.