something for brake fluid ...

you can add the brake fluid by yourself. Be very careful not to spill it since the fluid is very corrosive and may damage the paint of your car.

the brake fluid is designed to use to push the brake pads based on the hydraulic theory. The fluid is not compressible and is high-heat resistent.

you don't need to change all if you don't have to.

Don't add it too much. As you change the brake pads, the push pistons are pushed back and the fluid may overflow.