Disc brakes feature brake pads that open automatically when the rider's hand leaves the brake lever. However, if the pads are to operate smoothly, the calipers must be serviced. If the pistons of dual and opposing four-piston calipers are out of sync, piston cleaner can be used to fix this.
When removing the pistons from the brake calipers, if the pistons are not removed smoothly using pliers or air compressor gas, you can connect the brake fluid line directly to the brake master cylinder and use tap water to push it out.
It's a bit of a waste to use brake fluid just to remove the piston, and you don't need to worry about dirty paint if you use tap water. Remember to wait until the water in the brake master cylinder and fluid lines has completely dried out before putting it back in after use.
When your hand leaves the brake lever, the piston that has been pushed into the brake master cylinder returns to its original position via the reset spring and the brake fluid that has been pushed into the side of the caliper by the brake line returns to the brake master cylinder reservoir. The pressure on the brake fluid inside the caliper will then disappear and as the brake fluid returns to the brake master cylinder, the piston holding down the lining will also return to the caliper. As the brake fluid returns to the brake master cylinder, the piston holding the pads in place returns to the caliper. When the force applied to the pads holding the discs is reduced, the braking force automatically disappears.
Even if you don't even notice this when you are riding, these parts will act individually every time you release or hold the brake lever. If you are using a hydraulic disc system, the most important part of this chain of action is the roll back of the caliper seal. When the seal is rolled back, the deformed seal is restored to its original shape and the seal attached to the brake caliper piston prevents the brake fluid from leaking out of the caliper and also allows the forward piston to return to its original position.
When the gap between the pads and the discs increases beyond the deformation of the oil seal, the piston automatically advances, keeping the distance between the pads and the discs at an appropriate value, and this is the characteristic of the disc brake system. Because of this feature, motorbikes with disc brakes do not need to adjust the clearance as drum brakes do.