Table of contents: Front Disc Brake Pads - Checking ↓ Rear Drum Brake Linings - Checking ↓
Front Disc Brake Pads - Checking
The brake pads of the front disc brakes are usually sufficient for a mileage of 50,000 km. At the same time, we recommend not to neglect regular visual inspection of their condition. The permissible residual thickness of the brake pads is 2 mm. The thickness of the support metal plate on which the pad is attached is not taken into account. This plate has a thickness of about 5 mm. Visual inspection of the thickness of the pads is performed with the wheel cap removed.
1. Illuminate the brake pads with a flashlight through the hole in the wheel rim and assess the condition and thickness of the brake pads. More accurate is to check the brake pads on the removed wheels.
2. Measure the thickness of the inner and outer brake pads using a ruler.
When removing the wheels to check the condition of the brake pads, the brake discs should also be checked at the same time
The surface of the brake discs should not have grooves formed as a result of friction from dirt. These grooves lead to premature wear of new brake pads and reduce braking efficiency. Grooved brake discs can be finished by grinding if they are not worn out and their thickness allows this. Blue discoloration of the metal of the brake disc can be ignored.
The brake discs should be replaced on both wheels at the same time. Replacing the brake disc on only one wheel may result in uneven braking of the wheels.
When replacing brake pads, it is essential to replace the brake pads of both wheel brake mechanisms.
Checking the condition of the rear wheel disc brake pads is done in the same way as the front ones. The residual thickness of the brake pads, excluding the support plates, should not be less than 2 mm.
Rear Drum Brake Linings - Checking
When determining the residual thickness of the brake linings of rear drum brakes, only the thickness of the front shoe is measured because it wears out to a greater extent.
3. Place the rear of the vehicle on jack stands.
4. Remove the plastic cap from the corresponding rear wheel.
5. Shine a flashlight into the inspection hole in the caliper housing. The remaining thickness of the brake pads without the backing plate should be at least 2.5 mm.
Brake pedal free play - check
6. Press the brake pedal by hand. The free travel of the pedal should be a maximum of ⅓ of the possible pedal travel. A longer free travel means that the linings have worn down to the maximum permissible thickness and must be replaced.
Extended brake pedal travel can also be caused by sticking brake pads or calipers. Air entering the brake system also leads to increased free travel of the brake pedal.
Parking brake lever free play - check
The parking brake should operate when the parking brake lever is set to the third tooth of the sector. When the parking brake lever is tightened to the fourth tooth of the sector, the rear brakes should be locked.
In vehicles with rear disc brakes, the free travel of the parking brake lever depends on the operation of the adjustment mechanism. However, in general, the following rule can be used as a guide. If the travel of the parking brake lever is increased, this is evidence of wear of the rear brake pads or the result of stretching of the drive cables.
As a rule, parking brake adjustment is only necessary when replacing the cable or rear brake shoe linings.
Brake booster vacuum - check
7. Turn off the engine.
8. Press the brake pedal 10 times.
9. Leave the brake pedal depressed and start the engine. If the brake booster is normal, the brake pedal will move down a little more when the engine is running. If this does not happen, the booster is faulty. The most likely cause of the defect is a leak in the low-pressure air supply hose from the intake manifold, a faulty check valve on the low-pressure hose, wear of the rubber sealing ring between the master cylinder and the booster, or the booster cuff.
Brake fluid - replacement
Moisture absorbed by brake fluid not only lowers its boiling point, but also leads to corrosion of brake cylinders and pipes. For this reason, the maintenance plan provides for replacement of brake fluid every two years.
To replace the brake fluid you need two liters of fresh fluid.
10. Drain all the brake fluid from the expansion tank, using a siphon or an old oil can. Once all the fluid has been drained from the expansion tank, pour new fluid into the tank.
11. Bleed the system until fresh brake fluid comes out of the bleeder nipple. To ensure that all used brake fluid is removed, drain 500 cm³ of brake fluid from each bleeder nipple when bleeding.
The order of pumping is: rear right, rear left, front right and lastly front left.
Checking the condition of the tires
The condition check is best performed with the vehicle on jack stands, such as when changing the oil at a petrol station.
12. Turn each wheel completely.
13. Remove any foreign bodies that have entered the tread pattern profile using a small screwdriver. Work carefully so as not to damage the tread.
The tire tread pattern, in accordance with regulatory documents, must have a depth of at least 1.6 mm across the entire width of the tire.
The wear is checked by the raised areas in the tread groove. They are marked on the sidewall of the tire with the letters TW1 (tire wear indicator).
When these marks are at the same height as the ribs of the tread pattern, this means that the residual profile value is 1.6 mm, and the tire needs to be changed.
The nature of the tire tread wear can be used to judge whether the tire is being used correctly and whether the wheel is properly balanced.
Wear on the outside of the front tyres is normal and is caused by the increased stress on these areas of the tyre surface when cornering.
One-sided wear of the profile indicates incorrect installation of the wheels.
Severe wear in the middle of the profile indicates increased air pressure in the tires.
Severe wear on the outer sides of the profile indicates that the vehicle has been operated for a long time with insufficient air pressure in the tires.
Uniform "bald spots" in the tire profile indicate a faulty shock absorber strut.
Uneven wear in certain areas of the tread indicates an imbalance in the wheel or a defect in the tire frame.
If there is a spot on the tread with a heavily worn pattern, this is the result of braking with a locked wheel (only on cars without ABS).
Note: When dismantling a balanced wheel to perform any work, it is recommended to first mark its position on the hub. Subsequently, the wheel can be installed in its original place and its balancing will not be required.
Wheel replacement
Changing a wheel on the road can be a challenge, especially if the wheel bolts are rusted. Use a piece of pipe as a lever to increase the force when loosening the bolts.
14. Apply the parking brake and engage 1st gear or reverse.
15. Turn on the hazard warning lights and place a warning triangle on the road.
16. Place rocks or pieces of wood under the wheels on the opposite side of the car to prevent the car from rolling away
17. Remove the cap.
18. Loosen the wheel mounting bolts by one turn.
19. Place a car jack under the mark on the edge of the body and lift the car.
20. Completely unscrew the mounting bolts.
21. Remove the flat tire and install the spare.
22. Apply a thin coat of grease or oil to the bolt tapered surfaces and threads and tighten the bolts evenly in a crisscross pattern, turning the wheel back and forth to ensure that the wheel is centered on the hub.
23. Lower the car and tighten the bolts to 110 Nm.
24. Put on the cap. Don't forget to insert the tire valve into the slot on the cap.
After changing the wheel, after driving a few kilometers, you should check that the bolts are tightened correctly.
Wheel balancing
Wheel imbalance is felt by vibrations in the steering wheel or the front of the car. Both types of vibration occur only at a certain speed. The reason is the uneven distribution of mass around the wheel. Therefore, wheels must undergo static and dynamic balancing. There are two methods for this purpose.
- The wheel is removed from the car and mounted on a balancing machine. The machine is turned on to determine the imbalance, which is corrected using lead weights.
- Precise wheel balancing is performed on a special stand. In this case, the imbalance of the wheel hub and brake disc is compensated. The wheels of front-wheel drive cars are driven from the engine to the required rotation speed to determine the residual imbalance, which is also eliminated using lead weights.
As for all-wheel drive vehicles, they are mounted on trestles so that all wheels rotate freely.
(The article is a reprint of material from: AUDImanual)