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Features of operation of brake system

Experienced motorists know that rainy summer braking system of the machine for some reason becomes more whimsical than, for example, in the heat. Trouble can begin right in tarazhe. You come early in the morning to his car, got behind the wheel and just touched the brake pedal - as it is, darling, fell limply to the floor. What a shame - because yesterday pedal was habitually rigid. However, the free wheeling was more usual, but you probably did not attach any importance to this. And in vain ... Now in the upset feelings you get out of the car and looks at him. So it is: on the back of one of the support panels of the rear wheels and brake drum extensive smudges of brake fluid. Then opened the hood and looking into the reserve tank with brake fluid, make sure that almost all of it flowed. Things are bad. About the trip can not be considered. Repair needed. But because of what exactly is there? And why so suddenly?

Well, firstly, not so suddenly. Chances are, you just do not notice the start of leakage before. Secondly, let us look at the reasons for this failure of the brake actuator, unfortunately, characteristic for cars of respectable age.

Recall the construction of the foundations of modern hydraulic drive of brake mechanisms. It includes a master cylinder with a reserve tank for brake fluid, brake booster, double-pressure regulator rear brake mechanisms and brake pedal. Hydraulic brake actuator is divided into two independent circuits. One circuit provides the right front and left rear wheels, the other - the left front and right rear. Such diagonal scheme of division of contours today widespread in cars of leading world companies. It greatly improves traffic safety in comparison with previously used scheme.

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The fluid pressure in the pipes and wheel brake cylinders under severe braking can reach 10-12 MPa (100-120 kg / ^cm 2). Therefore, seal all conduits, and even more, drive moving parts - a key requirement for the design. And if the fixed connection of pipelines is relatively easy to seal, the moving parts - piston master cylinder, wheel cylinders and the amplifier - to condense complicated.

The world practice knows the car manufacturers today are two kinds of rubber seals cuff: solid, bowls, without a central hole and a rubber washers with a convex outer surface. The first are rarely used today, the second - the most common, as cheaper and processing.

Design and operation of such seals by the example of a typical design of the master cylinder tandem. In his case there are two movable piston, creating two independent brake actuator circuit. The tightness of the back of the piston is provided with a rubber cuff. However, the cuffs will not prevent the liquid from leaking while the first seal - cuffs - wear. It is more boot that protects the piston against dirt from outside. Main seal pistons - cuffs. Prior braking pistons are stationary, and the cuff is not in contact with the end faces thereof, as held by spacing rings in abutting mounting bolts. In this position of the piston of the cylinder filled with hydraulic fluid from the reserve tank passing through the gaps between the seal and the piston. Main cuffs have a cross-section of the toroidal shape. Their outer diameter in the free state, slightly larger than the inner diameter of the brake cylinder. Therefore, if the cuff is not exposed to brake fluid pressure, it is only its average outer band in contact with the cylinder bore. That is why at the slightest deterioration of the outside of the cuff - let it be just one single risk - the liquid begins to flow out. And this, mind you, when the vehicle is stationary.

Pressing the brake pedal moves the piston forward and into contact with the end surface of the cuff. Contact reliability provided by a spring. From that point a message with the internal cavity reserve tank is stopped and the pressure in the cylinder begins to rise in the piping. Under the influence of fluid pressure in the cuff is distributed radially and firmly pressed against the mirror cylinder entire outer surface.

Second, the floating piston of the master cylinder is moved by fluid pressure, and the work of his collar is no different from that described. Similarly, working and cuff wheel brake cylinders. Due to the fact that in the intervals between brake applications, the contact area with cylinder sleeves minimal surfaces provide good grease cylinder with brake fluid. In addition, the start of braking is smooth, without jerks, which significantly improves the comfort and safety of the vehicle.

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The cavities formed between the parts of the master cylinder in between braking, have a volume completely providing compensation for thermal expansion of the brake fluid after repeated or prolonged braking, thereby eliminating the self-jamming the brakes. In addition, the free circulation of the liquid during heating and subsequent cooling system reduces clogging cylinders dirt and facilitates spontaneous removal of air bubbles from the brake master cylinder (of wheel cylinders air, unfortunately, he is not deleted - need a pumping system).

So what to do if the brake flowed? Let's start with the most common - to repair the wheel brakes. Cylinders rear drum brakes front disc flow increasingly (for disk jamming characteristic pistons instead of cuff). We offer a sequence of actions, which performance quite a force to any driver and does not require any special tools.

First of all, you need to loosen the bolts or wheel nuts, and then pick up the car jack and having substituted under it a safety stand, remove the wheel. Next, remove the brake drum. In many cars, he does no longer fixed and after the dismantling of the wheels can be easily removed by hand. In the case of attachment, except for wheel bolts, screws and guide even the removal of the drum is a certain difficulty. Often after untwisting of these screws can not remove the drum with centering planting belt axis. Especially if it is from the factory never filmed, as they say, has become attached.

It is, of course, try to tighten the screws remove guide into the threaded hole of acting as a stripper. Often, however, it only leads to the tearing away of the thread, and the drum - not stir. Then resorted to the next step. Securely fixed linings wheels standing on the ground, start the engine, downshift and when the drum rotates at a low speed sharply push the brake pedal. If hydraulic drive does not work dramatically inhibit the parking brake. Typically, when such dynamic loading axis rotates in an opening of the drum, and then remove the drum has no difficulty.

After removing the drum relax the parking brake cable and remove the tip from the lever. Pulling out of the hole cotter pin, remove the lever (on some models of cars removal of this lever is not required). Then release the reference shield brake pads by removing the spring guide, and start lifting pads themselves. A common mistake here is the attempt to remove first of all the upper spring, constricting shoes. It makes it very difficult, especially with bare hands or with only one screwdriver. Much easier to remove first the lower, weaker spring, and then push the lower ends of the pads from the slots support and move our shoes on and operating them as long arms, stretch top spring and remove raszhimnuyu bar parking brake, and then do pad with support slots piston brake cylinder. It is important not to damage the protective cap rubber cylinder.

Now it is possible to disassemble the wheel brake cylinder. Here I would like to give this advice. If you do not mind the old brake fluid, you can immediately remove the protective caps and with the help of a soft material punch press out of the cylinder piston assembly with automatic regulation of clearance items. This will result in the liquid on the ground. If the fluid is maintained, better to disconnect the tube from the wheel cylinder and to muffle its wooden stopper. The cylinder is removed from the board and continue to support its dismantling on a workbench. Use a screwdriver to turn the piston, unscrew from it the persistent screw and remove cuffs with support cup and crackers. Then disconnect the thrust washer and screw.

After dismantling all details wipe and carefully inspect the surface of the cylinder and cuffs. Mirror cylinder must be absolutely clean and smooth, with no rough edges and scratches, and the outer surface of the cuff should not be even the slightest irregularities.

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Minor defects on the mirror of the cylinder can be removed by lapping, taking into account that an increase in its diameter is rather undesirable. Cuffs also necessary to change to a new, even if the old have no visible wear. It is also necessary to check for damage to the protective rubber caps of the cylinder and, if necessary, replace them with new ones.

Before assembly must be liberally grease all parts with brake fluid, and perform the operation in reverse order. After assembly, check the movement of the pistons in the cylinder, compressing the pistons by hand. They should move without jerks and jamming.

The general assembly of the brake mechanism is carried out in the reverse order specified.

The last thing you need to do - is to fill the brake fluid drive and remove it from the air. To do this, fill the reserve tank with a fresh brake fluid up to the mark "MAX". Then proceed to the removal of air from the wheel cylinder of the outermost wheel. To do this, put on the head of the union of a rubber hose, and its free end is immersed in a glass container partially filled with brake fluid. Very depressing the brake pedal 3-5 times at intervals of 2-3 seconds, wrench fitting half a turn, continuing pressing the pedal (this requires two people). Air brake fluid will come out of the tube. Further, lowering pedals, wrap the union. Repeat the entire operation until then, until out of the tube does not cease to leave air bubbles, the pedal does not become "rigid". The same operation is repeated for the other wheels.

It should be noted that the pumping system can be performed alone. To do this, all of the preparatory operations. However, after loosening the union need to get behind the wheel and the brake pedal 3-4 times. Then, without removing the tube and removing it from the liquid in the vessel, fitting wrap to failure. Then again get behind the wheel and check the "rigidity" of the pedal. If you move it even slightly decreased, proceed to the same operations serially with other wheels of the car. When pumping the last pedal brake cylinder should be as rigid, and its progress - more than half its full stroke. When removing the air is absolutely necessary to follow EA reduction in the fluid level in the reserve tank and refill it constantly. Of course, alone described operations of pumping the brake actuator will take longer than two. However, if you have time, make sure to deal with this type of repair can be completely alone.