There is no bleed screw - this is a typical sensor failure. Buy a new coolant tank.
Sensor checks the level, not the constituents of the fluid so you can have it full of beer - it dopesn't care as long as it is full.
Don't forget that 100% and anti-freeze mixture will freezebefore a 50/50 mix. I can't remember why, I did learn why a long time ago in advanced chemistry, but have since forgotten, because I just know that it does.