SHIMANO Brake Lever (Hydraulic Disc Brake) BL-MT501 Left Black.

The BL-MT500 brake lever offers high-performance SERVO WAVE for mountain bikes and I-SPEC II compatibility. 

Tapered and stable
Smooth and durable master cylinder design
Reliable power and a stiff brake design are ideal for all types of riding from XC to trail.
SERVOWAVE high-power braking system