Fig. 8.11. Front wheel brake mechanism with ventilated discs: 1 – bracket; 2 – caliper bracket; 3 – bleed nipple with protective cap; 4 – inner shoe; 5 – connection of brake pad wear sensor
For this operation, it is necessary to remove the wheels. The front wheels are equipped with ventilated brake discs, which, thanks to this design, dissipate heat from friction well (Fig. 8.11). The rear wheels are equipped with solid discs (without ventilation holes). When checking the pads, you should always inspect the brake discs, at least when replacing the pads. Discs with scratches, grooves from wear, in addition to deterioration of braking characteristics, lead to increased wear of the pads.
Sequence of works
1. The bluish color of the disc is not a sign of malfunction.
2. Pay attention to the scratches and wear marks on the disc. They appear from dirt or excessively worn pads. If the scratch depth is more than 0.5 mm, the disc needs to be replaced. Ventilated discs with thread-like scratches up to 25 mm long may not need to be replaced. However, if they increase, replacement is required.
3. Check the disc runout - it should not touch the pads when they are slightly brought together.
Fig. 8.12. Measuring the thickness of a disk using two coins (arrows)
4. Check the thickness of the disc in several places. It is best to measure with a caliper, having first placed two coins under its jaws on the disc (Fig. 8.12) – if the disc is worn, the measurements will be more accurate. Do not forget to subtract the thickness of the coins from the result obtained.
5. Brake discs cannot be processed.
6. Worn discs should always be replaced in pairs.
Brake disc sizes
The dimensions of brake discs for vehicles with diesel engines are presented in Table 8.1.
Table 8.1. Dimensions of brake discs for vehicles with diesel engines
The dimensions of brake discs for cars with petrol engines are presented in Table 8.2.
Table 8.2. Dimensions of brake discs for cars with gasoline engines
Recommendations for maintenance and repair of the brake system
1. If you open a closed hydraulic brake system somewhere, the working fluid will leak out. This will interfere with the work, and then you will need to top up the brake fluid. You can avoid this as follows: before starting work, first open the nipple for bleeding the brake mechanism of the corresponding wheel. Press the brake pedal and hold it. To do this, you can use a wooden stick of appropriate length, which should be clamped between the pedal and the driver's seat. The pistons in the master cylinder in this position block the holes for supplying brake fluid, so it does not leak out.
2. When repairing brake calipers or pistons, never use oil or any other substance for lubrication. Brake fluid should not come into contact with them. In addition, the lubricant will become more liquid when heated and may get onto the working surfaces of the brake mechanism. Special lubricants are required, which are usually included in the spare parts kit. When installing a piston in a brake cylinder, for example, a special paste for a brake cylinder (from ATE) is used. This substance is also applied to the inside of the new protective boot.
3. A brake caliper that was completely free of brake fluid during disassembly may cause certain difficulties when bleeding the system. Even if you press the brake pedal repeatedly, the fluid may not flow into it. In this case, proceed as follows: after installing the caliper, do not secure the upper end of the brake hose to the brake system pipeline. Pump brake fluid through the open bleed nipple until it begins to flow out of the upper end of the brake hose. Then secure the brake hose and screw on the bleed nipple. Then bleed the system. To pump in brake fluid, use a new plastic container filled with brake fluid and connected to the bleed nipple through a transparent hose. In this case, it is necessary to lift the container and pump brake fluid without air bubbles into the brake mechanism.
4. Be careful when handling brake fluid. It is aggressive towards paintwork and is very toxic. Do not allow it to get into your mouth or into open wounds.
5. Brake fluid drained from the system cannot be reused. Do not use fluid from a container that has been open for a long time. Used brake fluid must be disposed of properly.
6. After working with the brake hydraulic drive, check the pressure: to do this, press the brake pedal hard several times. Then, with the engine running, press the pedal hard again (200–300 N). The pressure in the system should be maintained, i.e. the pedal resistance should not decrease.
7. Rubber parts of the brake system that have been contaminated with oil must be replaced.
Caution! After completing the work, press the brake pedal several times to set the pads to the normal position. If this is not done, the brakes will not work during the first braking. It is necessary to carefully run in the new brake pads. To do this, lightly press the pedal several times to brake the car from a speed of 80-40 km/h.