Hydraulic disc brakes feature a line filled with hydraulic fluid, which connects the brake lever to the calipers with the piston and brake pads inside. When the brake lever is squeezed, the hydraulic fluid compresses the piston and causes the brake pads to pinch the rotor on the wheel. This creates friction, causes the wheel’s movement to slow down and stop. Disc brakes need to be bled as part of their maintenance, with the hydraulic fluid being replaced when needed.