BMW 3 Series E30

since 1983-1994 release

Repair and operation of the car

Skilled motorists know that in the rainy summer the brake system of the car for some reason becomes more whimsical, than, for example, during a heat. Troubles can directly begin in a tarazha. You came early in the morning to the car, took the wheel and only touched a brake pedal, – as it, darling, powerlessly failed to the floor. And that annoyingly – still yesterday the pedal was habitually rigid. However, the free wheeling was more usual, but you probably did not attach it significance. And in vain... Now in the upset feelings you go out of the car and look under it. Indeed: on the back of one of basic boards of back wheels and on the brake drum extensive smudges of brake fluid. Having opened then a cowl and having glanced in a reserve tank with brake fluid, you are convinced that honor all it flowed out. Affairs are bad. About a trip out of the question. Repair is necessary. But because of what it occurred? And why so suddenly?

Well, first, not so suddenly. Most likely, you just did not notice the beginning of dribble earlier. And secondly, let's understand the reasons of such failure of the brake drive, unfortunately, of respectable age, characteristic of cars.

Let's remember bases of a design of the modern hydraulic drive of brake mechanisms. It turns on the main brake cylinder with a reserve tank for brake fluid, the vacuum amplifier, the double-circuit 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 wheels, another – left forward and right back. Such diagonal scheme of division of contours is widespread in cars of the leading world firms today. It considerably increases traffic safety in comparison with earlier applied schemes.

Reference

Liquid pressure in pipelines and wheel brake cylinders at intensive braking can reach 10–12 MPas (100–120 ^kg/cm2). Therefore consolidation of all pipelines, and furthermore, mobile parts of the drive – the major requirement to a design. And if to pressurize motionless connections of pipelines rather easily, then mobile parts – – it is more difficult to condense pistons of the main brake cylinder, wheel cylinders and the amplifier.

The world practice of producers of cars knows two types of rubber sealants of cuffs today: continuous, bowl-shaped, without the central opening and in the form of rubber washers with a convex external surface. The first are applied seldom today, the second – are most widespread as cheaper and technological.

We will consider a design and the principle of work of such consolidation on the example of a typical design of the main brake cylinder of tandem type. In its case two mobile pistons creating two independent contours of the brake drive are placed. Tightness of a back part of the piston is provided to rubber cuffs. However this of cuffs will not prevent a liquid effluence outside if the main consolidation – cuffs – wore out. It is rather the boot protecting the piston from dirt from outer side. The main consolidation of pistons – cuffs. Prior to braking pistons are not mobile, and cuffs do not adjoin to their face surfaces as are kept by the expansion rings resting against adjusting bolts. At such position of pistons of a cavity of the cylinder are filled with the brake fluid passing from a reserve tank through gaps between cuffs and the piston. The main cuffs have the section of a toroidal form. Their outer diameter in a free state slightly exceeds the internal diameter of the brake cylinder. Therefore if cuffs are not exposed to pressure of brake fluid, then only its average external corbel adjoins to a cylinder mirror. For this reason at the slightest wear of outer side a cuff – let it will be all unique risk – liquid will begin to follow outside. And it, notice, at the parking of the car.

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

The second, floating, the piston of the main brake cylinder moves under pressure of liquid, and work its cuff differs in nothing from described. In the same way also cuffs of wheel brake cylinders work. Because in intervals between braking the area of contact of cuffs with cylinders is minimum, good lubricant of surfaces of the cylinder is provided with brake fluid. Besides, the beginning of braking occurs smoothly, without breakthroughs that significantly improves comfortableness and traffic safety of the car.

Reference

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

So to undertake if brakes began to flow? Let's begin with the most widespread – from repair of wheel brake mechanisms. Cylinders of back drum brake mechanisms leak more often than lobbies disk (jamming of pistons, but not course of cuffs is characteristic of disk). We offer the sequence of actions which performance quite in power to any driver and does not demand any special devices.

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 the brake drum. On many cars he is any more fixed by nothing and after dismantle of a wheel easily acts manually. In case of fastening, except wheel bolts, also the directing screws removal of a drum makes a certain difficulty. Often after untwisting of these screws the drum does not manage to be removed from the aligning landing corbel of a half shaft. Especially if he after factory assembly never acted and as speak, stuck.

It is necessary, of course, to try to twirl the removed directing screws in special carving openings, working with them as a stripper. However often it leads only to breaking of a carving, and the drum – will not stir. Then resort to the following operation. Having reliably recorded linings the wheels standing on the earth, start the engine, include a low gear and at rotation of the drum with a small speed sharply press a brake pedal. In case the hydraulic drive does not work, sharply brake the parking brake. As a rule, at such dynamic loading the half shaft is turned in a drum opening then it is possible to remove a drum already.

After removal of a drum weaken a cable of the parking brake and remove its tip from the lever. Having taken out the forelock from a finger opening, remove the lever (on some models of cars removal of this lever is not obligatory). Further exempt brake shoes from a basic board, having removed the directing springs, and start removal of blocks. A widespread mistake at the same time is the attempt to remove first of all the top spring which is pulling together blocks. It is very difficult to make it, especially barehanded or with only one screw-driver. It is far simpler to remove at first the lower, weaker spring, then to put forward the lower ends of blocks from grooves of a support and, having displaced a little block on itself and working with them as long levers to stretch the top spring and to remove a raszhimny level of the parking brake, and then and blocks from basic cuts of pistons of the brake cylinder. It is very important not to damage rubber protective caps of the cylinder at the same time.

Now it is possible to disassemble the wheel brake cylinder. Here it would be desirable to give such advice. If you do not grudge old brake fluid, it is possible to remove protective caps at once and by means of a drift from soft material to vypressovat from the cylinder pistons assembled with details of automatic control of a gap. Liquid at the same time will pour out on the earth. If liquid has to be kept, better to disconnect a tube from the wheel cylinder and to muffle it a wooden stopper. After that the cylinder is removed from a basic board and continue its dismantling on a workbench. By means of the 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 the surface of the cylinder and cuffs. The mirror of the cylinder has to be absolutely pure and smooth, without рисок and roughnesses, and on an external surface of cuffs should not be even the slightest roughnesses.

Reference

Minor defects on a mirror of the cylinder can be eliminated with grinding in, considering at the same time that increase in its diameter is very undesirable. Cuffs should be changed on new even if old have no visible wear. It is also necessary to check whether protective rubber caps of the cylinder are damaged and if necessary to replace them new.

Before assembly it is necessary to grease plentifully all details with brake fluid and to execute the specified operations upside-down. After assembly it is necessary to check movement of pistons in the cylinder, having squeezed pistons hands. They have to move without breakthroughs and jammings.

The general assembly of the brake mechanism is carried out to the sequences, return specified.

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

It is necessary to notice that pumping of system can be executed also to one. For this purpose it is necessary to execute all specified preparatory operations. However after untwisting of the union it is necessary most to take the wheel and to press a brake pedal 3–4 times. Then, without picking up the phone and without taking out it from liquid in a vessel, to wrap the union to the full. Then to take the wheel again and to check "rigidity" of a pedal. If its course decreased any, to start the same operations serially with other wheels of the car. When pumping the last brake cylinder the pedal has to be the most rigid, and its course – more than 1/2 full speeds. During removal of air it is absolutely necessary to watch эа decrease in level of liquid in a reserve tank and to constantly add it. Of course, the operations on pumping of the brake drive alone described will take more time, than together. However then you are once again convinced that it is possible to cope with this type of repair quite independently.