The ABS system was originally installed only on quattro models and vehicles with a six-cylinder engine. Starting from January 1993, all AUDI 80 models are equipped with an ABS system.
When you press the brake pedal, a pushrod connected to the pedal presses on two pistons located one behind the other in the master brake cylinder. In this case, the brake fluid is squeezed out and the resulting pressure is transmitted through the hoses and pipelines of the brake system to the working cylinders of the brake mechanisms of the wheels. The pistons of the working cylinders press the brake pads against the brake discs. On the rear wheels of AUDI 80 cars manufactured up to 10/92 with a 66 kW engine without ABS, instead of brake discs, brake drums are installed. All other versions, including models with a 66 kW engine with ABS, have disc brakes front and rear.
The brake fluid pressure is distributed through two independent brake circuits arranged diagonally. This means that the brake circuit is formed by the front and diagonally opposite rear wheels. This ensures that the car stops with sufficient efficiency in case of failure of one of the circuits of the working system. In this case, more effort is needed on the brake pedal, and the braking distance will be longer.
The parking brake acts on the brake mechanisms of the rear wheels through cable rods, regardless of the type of brake mechanism.
Brake fluid
AUDI 80 vehicles use FMVSS116DOT4 specification brake fluid. All fluids of the specified specification can be safely mixed with each other.
Brake fluid is poisonous and corrosive to vehicle paintwork. It does not freeze even at temperatures of -40°C and has a boiling point of about 260°C. The disadvantage of brake fluid is its increased hygroscopicity, which leads to a decrease in the boiling point. With 2.5% water in the brake fluid, its boiling point is only 150°C. This is dangerous, especially when the brakes are heavily loaded. The moisture absorbed by the brake fluid not only lowers its boiling point, but also leads to corrosion of the brake cylinders and pipelines. For this reason, the maintenance plan calls for changing the brake fluid every two years.
To replace the brake fluid, you need two liters of fresh.
1. Drain all the brake fluid from the expansion tank, taking it with a siphon or an old oiler. As soon as all the liquid from the expansion tank has been drained in this way, fill the tank with new one.
2. Bleed the system until fresh brake fluid comes out of the bleeder port. To ensure complete removal of used brake fluid, drain 500 cm3 of brake fluid from each bleeder port when bleeding.
Bleeding order: rear right, rear left, front right, and last front left.
Attention! Used brake fluid must be disposed of.
A low brake fluid level is signaled to the driver by a warning light on the instrument panel. However, we recommend regular visual inspection of the system.
3. Open the hood and, if necessary, wipe the expansion tank with brake fluid with a rag.
The expansion tank is made of transparent material, which makes it easy to determine the level of brake fluid, which should be between the MIN and MAX marks (see illustration 1.0).
When the brake pads are worn, the level of the brake fluid decreases, and after installing new ones, its level is slightly higher, which is allowed. However, if the brake fluid level has dropped to the MIN·mark or lower, then you need to urgently look for a leak. There should be no other reason, because the liquid does not evaporate and is not consumed.
In order to quickly determine the location of a possible leak, it is desirable that the bottom and the components and parts located there are dry. Wet areas suggest fluid leakage.
4. Check all the joints of the brake system, as well as the calipers and shields of the brake drum mechanisms, behind which are the wheel brake cylinders.
5. Make sure the brake hoses are not wet, swollen or frayed. Otherwise, replace the corresponding hose.
6. Do not clean the brake pipes with a screwdriver, sandpaper or wire brush because these pipes have a protective coating. Such cleaning will ensure the formation of rust.
7. Apply an anti-rust primer to damaged areas.
8. Replace pipelines with corrosion pits and deflections.
9. Install new protective caps on the bleeder fittings if the previous ones are not available.
10. Check the brake system. To do this, press the brake pedal sharply. The resistance to pedal pressure should not decrease even after a few minutes. Otherwise, there is a defect in the protective cuff of the main brake cylinder. If the cuff of the main brake cylinder is defective, the level of brake fluid in the expansion tank does not decrease. However, pressurized fluid leaks to the back of the brake master cylinder piston.
Places of leakage of brake fluid on the cuffs of the pistons of calipers and wheel brake cylinders can be identified by dark spots on the surface of the brake shield and caliper.
Increased brake pedal travel may indicate that the brake pads have worn out and need to be replaced. Excessive brake pedal travel can also be caused by stuck brake pads or rusted brake drums on vehicles with this type of rear brake. Too much pedal travel is also observed when air enters the brake system.
11. Press the brake pedal by hand. The pedal free play should not be more than ⅓ of its possible total travel.
If you have doubts about your own abilities, we recommend that you contact a workshop to check the brake system.
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