It is the Jaguar disc brake pads that wear out over time during the braking process. This is no accident, but rather by design so the rotors are not damaged when the friction is being applied. On Jaguars, there are four sets of these disc brake pads, one for each wheel. As the disc brake pads wear down, the piston in the caliper moves out. This drops the fluid level in the master cylinder. If upon inspection of the master cylinder you feel should have some fluid added, it is time to check your Jaguar disc brake pads to see if the replacement of them is near. If you just add brake fluid, then when your next brake job occurs and the pistons are pushed back into the calipers, the fluid will overflow and will have to be cleaned up before the finish on your Jaguar is damaged. Brake fluid is known to eat paint. Another warning sign that the disc brake pads are in need of replacing is when the driver hears a grinding noise when the brake pedal is being applied. This is the metal to metal contact of the disc and the backing plate of the pads. It occurs when all of the pad is gone. This also damages the disc causing grooves to be ground into it creating an uneven surface. These grooves will then have to be removed with the use of a lathe before the new disc brake pads can be installed. If that is not done then the new pads will prematurely wear. To help prevent that type of damage from occurring, along with the expense of having the disc turned, it is advisable that all Jaguar owners keep an eye on the brake fluid level in their master cylinder. This is the best indicator that a brake job will be required.