Audi 100/A6

1990-1997 of release

Repair and car operation

14.16.1. Features of operation of brake system

THE GENERAL DATA

Skilled motorists know that rainy summer the brake system of the car for some reason becomes more whimsical, than, for example, in a heat. Troubles can directly begin in тараже. You have come early in the morning to the car, have sat down for a wheel and only have touched a brake pedal, – as it, darling, has powerlessly failed to the floor. Also that annoyingly – after all still yesterday the pedal was habitually rigid. However, the free wheeling was more usual, but you, possibly, have not given it value. And in vain... Now in the upset feelings you leave the car and look under it. Indeed: on the back party of one of basic boards of back wheels and on a brake drum extensive smudges of a brake liquid. Having opened then a cowl and having glanced in a reserve tank with a brake liquid, are convinced that almost all it has flowed out. Affairs are bad. About a trip cannot be and speeches. Repair is necessary. But because of what such has occurred? And why so suddenly?

Well, first, not too suddenly. Most likely, you simply did not notice the beginning подтекания earlier. And secondly, let's understand the reasons of such refusal of a brake drive, unfortunately, characteristic for cars of respectable age.

Let's recollect bases of a design of a modern hydraulic drive of brake mechanisms. It includes the main brake cylinder with a reserve tank for a brake liquid, the vacuum amplifier, a two-planimetric regulator of pressure in back brake mechanisms and a brake pedal. The hydraulic drive of brake mechanisms is divided into two independent contours. One contour ensures functioning right forward and left back of wheels, another – left forward and right back. Such diagonal scheme of division of contours today widespread in cars of leading world firms. It considerably raises traffic safety in comparison with earlier applied schemes.

Pressure of a liquid in pipelines and wheel brake cylinders at intensive braking can reach 10–12 Mpa (100–120 kg/sm '). Therefore consolidation of all pipelines, and furthermore, mobile parts of a drive – the major requirement to a design. And if motionless connections of pipelines герметизировать it is rather easy, mobile parts – pistons of the main brake cylinder, wheel cylinders and the amplifier – to condense more difficult.

World practice of manufacturers of cars knows today two kinds of rubber sealants of cuffs: continuous, чашеобразные, without the central aperture and in the form of rubber washers with a convex external surface. The first are applied today seldom, the second – are most extended, as cheaper and technological.

Design and principle of work of such consolidation we will consider on an example of a typical design of the main brake cylinder тандемного type. In its case two mobile pistons creating two independent contours of a brake drive are placed. Tightness of a back part of the piston is provided rubber манжетом. However this of cuffs will not prevent вытекание a liquid outside if the basic consolidation – cuffs – has worn out. It is faster duster, the protecting piston from a dirt from an outer side. The basic consolidation of pistons – cuffs. Prior to the beginning of braking pistons are motionless, and cuffs do not adjoin to their face surfaces as are kept распорными by the rings resting against adjusting bolts. At such position of pistons of a cavity of the cylinder are filled with the brake liquid which is passing from a reserve tank through backlashes between манжетом and the piston. The basic cuffs have section of the toroidal form. Their external diameter in a free condition hardly exceeds internal diameter of the brake cylinder. Therefore, if cuffs is not exposed to pressure of a brake liquid only its average external corbel adjoins to a cylinder mirror. For this reason at the slightest deterioration of an outer side a cuff – let it will be all unique risk – the liquid will start to follow outside. And it, notice, at car parking.

By pressing a pedal of a brake the piston moves forward and the cuff contacts with a face surface. Reliability of contact is provided by a spring. From this point on the message of an internal cavity with a reserve tank stops, and pressure and in pipelines starts to increase in the cylinder. Under the influence of this pressure of a liquid of cuffs it is distributed in a radial direction and reliably nestles on a mirror of the cylinder all external surface.

The second, floating, the piston of the main brake cylinder moves under the pressure of a liquid, and work its cuff differs nothing from the described. Also cuffs of wheel brake cylinders in the same way work. That in intervals between braking the area of contact of cuffs with cylinders is minimum, good greasing of surfaces of the cylinder is provided with a brake liquid. Besides, the braking beginning occurs smoothly, without jerks that essentially improves comfortableness and traffic safety of the car.

The cavities formed between details of the main brake cylinder in intervals between braking, have the volume completely providing indemnification of thermal expansion of a brake liquid at repeated or long braking, excluding thereby self-jamming of brake mechanisms. Besides, free circulation of a liquid at warming up and the subsequent cooling of system reduces probability of a contamination of cylinders a dirt, and also facilitates spontaneous removal of vials of air from the main brake cylinder (from wheel cylinders air, unfortunately, by itself will not leave – system prorolling is necessary).

So to undertake, if brakes have begun to flow? We will begin with the most widespread – from repair of wheel brake mechanisms. Cylinders of back drum-type brake mechanisms flow more often than lobbies disk (for the disk jamming of pistons, instead of course of cuffs is characteristic). We offer sequence of the actions, which performance quite under force to any driver and does not demand any special adaptations.

First of all it is necessary to weaken bolts or nuts of fastening of wheels, and then, having lifted the car a jack and having substituted under it a safety support, to remove a wheel. Further it is necessary to remove a brake drum. On many cars it more is not fixed by anything and after wheel dismantle easily acts in film manually. In a fastening case, except wheel bolts also directing screws drum removal makes certain difficulty. Often after untwisting of these screws the drum does not manage to be removed from an aligning landing corbel of a semiaxis. Especially if he after factory assemblage never acted in film and as speak, has become attached.

It is necessary, of course, to try to twirl the removed directing screws in special carving apertures, operating with them, as a stripper. However often it conducts only to carving breaking, and the drum – will not stir. Then resort to the following operation. Reliably having fixed linings wheels standing on the earth, start the engine, include a hill-climbing gear and at rotation of a drum with small speed sharply press a brake pedal. In case the hydraulic drive does not work, sharply brake a lay brake. As a rule, at such dynamic нагружении полуось it is turned in a drum aperture then to remove a drum any more does not make work.

After drum removal weaken a cable of a lay brake and remove its tip from the lever. Having taken out from a finger aperture шплинт, remove the lever (on some models of cars removal of this lever not necessarily). Further release from a basic board brake Pads, having removed directing springs, and start removal колодок. A widespread error thus is attempt to remove first of all the top spring which is pulling together Pads. To make it it is very difficult, especially barehanded or with only one screw-driver. Where easier to remove at first the bottom, weaker spring, then to put forward the bottom ends колодок from grooves of a support and, having displaced a little Pads on itself and operating with them as long levers, to stretch the top spring and to remove расжимную a lath of a lay brake, and then and Pads from basic cuts of pistons of the brake cylinder. It is very important not to damage rubber protective caps of the cylinder thus.

Now it is possible to disassemble the wheel brake cylinder. Here it would be desirable to give such advice. If it is not a pity to you an old brake liquid, it is possible to remove at once protective caps and with the help выколотки from a soft material выпрессовать from the cylinder pistons in gathering with details of automatic control of a backlash. The liquid thus will pour out on the earth. If the liquid has to be kept, better to disconnect a tube from the wheel cylinder and to muffle its wooden stopper. After that the cylinder remove from a basic board and continue its dismantling on a workbench. By means of a screw-driver turn the piston, unscrew from it the persistent screw and remove cuffs with a basic cup and crackers. Then separate a persistent ring and the screw.

After dismantling all details wipe and attentively examine a surface of the cylinder and cuffs. The cylinder mirror should be absolutely pure and smooth, without рисок and roughnesses, and on an external surface of cuffs should not be even the slightest roughnesses.

Minor defects on a cylinder mirror can be eliminated grinding in, considering thus that the increase in its diameter is rather undesirable. Cuffs should be changed on new even if the old have no visible deterioration. It is necessary to check up also, whether protective rubber caps of the cylinder are damaged and if necessary to replace their new.

Before assemblage it is necessary to grease plentifully all details with a brake liquid and to execute the specified operations upside-down. After assemblage it is necessary to check up moving of pistons to the cylinder, having compressed pistons hands. They should move without jerks and jammings.

The general assemblage of the brake mechanism spend to sequences, the return specified.

The last that it is necessary to make is to fill a brake drive with a liquid and to remove from it air. For this purpose fill a reserve tank with a fresh brake liquid to a label "MOVE". Then start removal of air from the wheel cylinder of the most remote wheel. For this purpose put on an union head a rubber hose, and its free end lower in the transparent vessel partially filled with a brake liquid. Sharply having pressed a pedal of a brake of 3-5 times with an interval in 2–3 with, unscrew the union on a half-turn, continuing pressing a pedal (two persons are for this purpose necessary). Air with a brake liquid will leave a tube. Further, lowering pedals, wrap the union. Repeat all operation until the tube will not cease to leave air vials, the pedal does not become "rigid". The Same operations repeat for other wheels of the car.

It is necessary to notice that system prorolling can be executed and to one. For this purpose it is necessary to execute all specified preparatory operations. However after union untwisting it is necessary most to sit down for a wheel and to press a pedal of a brake 3–4 times. Then, without removing a tube and without taking out it from a liquid in a vessel, to wrap the union to the full. Then again to sit down for a wheel and to check up "rigidity" of a pedal. If its course though has a little decreased, to start the same operations serially with other wheels of the car. At prorolling of last brake cylinder the pedal should be as much as possible rigid, and its course – more than 1/2 full speeds. At air removal it is absolutely necessary to watch эа decrease in level of a liquid in a reserve tank and constantly to add it. Certainly, alone described operations on prorolling of a brake drive will occupy more time, than together. However then you are once again convinced what to cope with this kind of repair it is possible quite independently.