My brake light comes on if you hit the brakes hard. Brakes are very touchy. Brake light does not come on if you just lightly touch them. I've bled the brakes several times. Brake fluid level is ok. How did you find out you needed to replace the master cylinder? Thanks
Brake light and parking brake are different indicator lights. You either have a bad master cylinder or a leak in the brake lines. It will come on during a turn because your brake fluid is just at the minimum to keep the float thing away from the contact, but when you turn, it drops as the fluid moves to the side, indicating low brake fluid.