[About Brake Caliper Fastening]
The firmness is the typical failure of brake calipers.
When you step on the brake pedal, the hydraulic piston of the brake caliper is extruded,
Release the brake pedal and the twisting rubber will recover and return the piston to its original position.
When dirt or rust adhere to the piston, the rubber (piston seal) will dull movement,
The worst of the case is that the brake pad is pressed onto the brake rotor, as it does not recover and the piston is left out.
These conditions are called brake caliper adhesion.
If you leave the brake caliper firmly in place, the brake will be left on while driving.
You must not touch it inadvertently as the brake area can get quite hot.
It is also very dangerous because the bond causes only one side of the brake to be locked and the brake is in a working condition.