Brake fluid is poisonous. It also dissolves paint a lot. See the Warning at the beginning of the Section Bleeding the hydraulic brake system.
Inspection
At certain intervals specified in Table of sizes and adjustment data at the end of the Manual, the brake lines and hoses should be inspected. With the vehicle jacked up and securely on axle stands, inspect the flexible hoses for cracks, worn outer sheaths, fluid leaks, chafing, or other damage. Hoses are an important and vulnerable part of the brake system, so the inspection must be very thorough. Metal pipes should be checked for deep cavities and corrosion. If the above defects are found on the hose or tube, replace them.
It is helpful to use a flashlight and a mirror to carefully check the hoses and tubes.
Replacement of flexible hoses
1. Clean any dirt from the ends of the hose. Have a suitable container ready to collect any spilled liquid when the hose is disconnected.
2. To disconnect the chassis side hose, loosen the nut on the metal brake tube while holding the hex barb on the end of the flexible hose (bottom right arrow) using a split cap («brake») wrench (top right arrow). If the nut does not give in, moisten it with oil. After disconnecting the hose from the line, remove the spring clip (left arrow) from the bracket and then disconnect the hose from the bracket.
3. To disconnect a flexible hose from a support, simply give screws of fastening. If the hose will be disconnected for some time, plug the nipple connections on the caliper to prevent dirt from entering.
4. Install in reverse order. Make sure the brackets are not damaged and the locknuts are securely tightened. If the spring clips are loose, replace them.
5. Once again, carefully check whether the components of the suspension or steering system are in contact with the hoses. During the check, ask the assistant to turn the steering wheel from lock to lock.
6. Bleed the brake system as described in Section Bleeding the hydraulic brake system.
Replacement of metal brake lines
When replacing brake lines, use only original parts - preferably from a BMW dealer.
Original BMW brake lines are supplied directly. To give them the desired shape, you will need a tube bender.
1. First remove the tube you are about to replace, place it on a clean workbench and measure carefully. Prepare a new tube of the same size and bend it to the shape of the old one.
Do not bend or damage the line. The bending radius must never be less than 14 mm. Make sure the protective coating on the new line is not damaged at the folds.
2. When installing a new brake line, make sure that it is securely supported by brackets, that its route matches the original, and that it is at a sufficient distance from moving and heating components.
3. After installation, check the brake fluid level in the master cylinder and add it if necessary. Bleed the brake system as described in Section Bleeding the hydraulic brake system and check the operation of the brakes before operating the vehicle. Make sure there are no fluid leaks.