Where caliper guide pin bolts are unscrewed, they have to be renewed. Start by chocking the front wheels and lifting up the stands and removing the rear wheels making sure that at least one of the wheel bolts is re-fitted back in the correct hub to hold the brake discs in place. Pull the handbrake lever and push the handbrake cable adjuster as far as it will go with both caliper handbrake levers touching their stops. Unscrew and loosen bolts of the caliper guide pin without allowing the guide pins to rotate, and dispose of the old bolts and install new ones when fitting back. Move the caliper off the brake pads then firmly attached it to the suspension strut using wire and ensure that the caliper is not hanging unsupported on the flexible brake hose. Take out the two brake pads of the mounting bracket of the caliper, and measure the thickness of each pad, rejection to be made of those that are worn to or below the desired minimum thickness, and of any that are oily or greasy. Should the pads still be serviceable clean them and check the caliper and piston and see whether they are damaged or not even leaking. It is possible that new brake pads should be installed, in that case, fully retract the piston into the caliper bore by turning it clockwise, but you have to check the level of fluid in the master cylinder reservoir to avoid spillage. Install the pads into the mounting bracket, be sure the pads are correctly positioned against the brake disc, and carefully slip the caliper back in place and install the new guide pin bolts, and screw the bolts to the required tightness. The same procedure is repeated with the other rear brake caliper and set up the rear brakes. With these models (that are not fitted with self-leveling rear suspension) position the pressure regulating valve lever all the way to the rear axle, start the engine and press the brake pedal to ensure the pads touch the discs and ensure that the discs are rotating freely and point the handbrake. On models with self-leveling rear suspension, lower the car to the ground, switch on the engine so that the suspension can stabilize and repeat the process to make sure the brakes work properly. In case of A6 models, start the engine, press the brake pedal, watch the rotation of the disc, set the handbrake and lastly look at the level of the hydraulic fluid.