Brake warning and ABS light both on

For the last 6 months, I've had a problem with both the brake system and abs warning lights coming on together.

They would come on in the morning on the way in to work, but wouldn't come back on if I turned the engine off for 10 mins or so. The ABS doesn't work when the lights are on, but does work fine when the lights are off.

I think this has been happening since I accidentally put oil in the brake fluid. The oil has since been flushed out but I think that the oil may have damaged the brake fluid sensor in the resevoir.

Would a faulty fluid sensor make the ABS light come on too though? And is there a way I can bypass the sensor to test it? i.e. shorting out the plug for it to see if turns the light off?