Refresh Brake Fluid with Clear Tubing After emptying the reservoir, fill it up with fresh fluid. From this onwards, the process is no different than bleeding brakes. To make the process easier ...

The brake fluid reservoir is usually plastic and clear or ... Wash your hands if exposed, and wipe it off of painted surfaces immediately. Fill to the maximum line. Secure the cap tightly.

but like brake calipers, sometimes it can spring leaks or otherwise fail, and it must be rebuilt or replaced. The master cylinder is topped by the brake-fluid reservoir, which must be kept full.