Befor you go rippin it apart make sure the sensor on the master cylinder is connected properly l, give it a wiggle or even take it off and put it back on. Quite often at work after a brake change the abs light comos on cos the sensors playing up.

ok so noob question master cylinder is behind the rotor somewhere yeah? id be looking for a cord coming to a sensor near the rotor?

Lol,
Pop the bonnet, in the back left corner on the firewall is a big round black thing ( brake booster) on the front of it is a tube looking thing with a few lines and a sensor and a resiviour on top full of brake fluid, this is you're master cylinder. Wiggle the wire plug on it.
If you changed rotors I assume you changes pads and if youdid that you would have had to push the pistons back, if you did this brake fluid would have p****d out Of the resiviour and all over you're paint, make sure you wash it off with ahitloads of water or you'll have no paint left

the sensors on the master are only for brake fail warning and cruise control shut off

If the brake fluid has never been topped up while the pads were wearing off, the fluid doesn't spill out of the reservoir.

I'd check the sensor at the brakes.

Preferably use metho if you have some on hand.