Things are bad. Travel is out of the question. Repair needed. But why did this happen? And why so suddenly?
Well, first of all, not so suddenly. Most likely, you just did not notice the beginning of leakage before. And secondly, let's look at the reasons for such a failure of the brake drive, which, unfortunately, is typical for cars of venerable age.
Let's recall the basics of the design of a modern hydraulic drive of brake mechanisms. It includes a master brake cylinder with a reserve tank for brake fluid, a vacuum booster, a dual-circuit pressure regulator in the rear brake mechanisms and a brake pedal. The hydraulic drive of the brake mechanisms is divided into two independent circuits. One circuit provides the operation of the right front and left rear wheels, the other - the left front and right rear. Such a diagonal circuit separation scheme is now widely used in passenger cars of the world's leading companies. It significantly improves traffic safety compared to previously used schemes.
Reference. Liquid pressure in pipelines and wheel brake cylinders during intensive braking can reach 10-12 MPa (100-120 kg/cm2). Therefore, the sealing of all pipelines, and even more so, the moving parts of the drive, is the most important requirement for the design. And if the fixed connections of pipelines are relatively easy to seal, then the moving parts - the pistons of the main brake cylinder, wheel cylinders and amplifier - are more difficult to seal.
The world practice of car manufacturers today knows two types of rubber seals for cuffs: solid, bowl-shaped, without a central hole and in the form of rubber washers with a convex outer surface. The former are rarely used today, the latter are the most common, as they are cheaper and more technologically advanced.
Let us consider the design and operation principle of such a seal using the example of a typical design of a tandem-type brake master cylinder. Its body contains two movable pistons that create two independent brake circuits. The tightness of the back of the piston is provided by a rubber cuff. However, this lip will not prevent fluid from leaking out if the main seal, the lip, is worn out. It's more like a boot that protects the piston from dirt from the outside. The main seal of the pistons is the cuff. Prior to the start of braking, the pistons are stationary, and the cuffs do not come into contact with their end surfaces, since they are held by spacer rings resting against the mounting bolts. With this position of the pistons, the cylinder cavities are filled with brake fluid passing from the reserve tank through the gaps between the cuff and the piston. The main cuffs have a toroidal section. Their outer diameter in the free state is slightly larger than the inner diameter of the brake cylinder. Therefore, if the cuff is not subjected to brake fluid pressure, then only its middle outer belt is in contact with the cylinder mirror. That is why at the slightest wear on the outer side of the cuff - let it be just one single risk - the liquid will begin to flow out. And this, mind you, when the car is parked.
When you press the brake pedal, the piston moves forward and comes into contact with the end surface of the cuff. Reliability of contact is provided by a spring. From this moment, the communication of the internal cavity with the reserve tank stops, and the pressure in the cylinder and in the pipelines begins to increase. Under the action of this fluid pressure, the cuff expands in the radial direction and is reliably pressed against the cylinder mirror by the entire outer surface.
The second, floating, piston of the main brake cylinder moves under fluid pressure, and the operation of its cuff is no different from that described. The cuffs of the wheel brake cylinders work in the same way. Due to the fact that in the intervals between braking, the contact area of the cuffs with the cylinders is minimal, good lubrication of the cylinder surfaces with brake fluid is ensured. In addition, the start of braking occurs smoothly, without jerks, which significantly improves the comfort and safety of the car.
Reference. The cavities formed between the parts of the main brake cylinder in the intervals between braking have a volume that fully compensates for the thermal expansion of the brake fluid during repeated or prolonged braking, thereby excluding self-jamming of the brake mechanisms. In addition, the free circulation of fluid during heating and subsequent cooling of the system reduces the likelihood of clogging of the cylinders with dirt, and also facilitates the spontaneous removal of air bubbles from the brake master cylinder (unfortunately, the air will not be removed from the wheel cylinders by itself - the system needs to be pumped).
So what to do if the brakes are leaking? Let's start with the most common - with the repair of wheel brakes. Rear drum brake cylinders leak more often than front disc brakes (discs are characterized by jamming of the pistons, and not the flow of cuffs). We offer a sequence of actions, the implementation of which is quite within the power of any driver and does not require any special devices.
First of all, you need to loosen the bolts or nuts securing the wheels, and then, having raised the car with a jack and substituting a safety stand under it, remove the wheel. Next, you need to remove the brake drum. On many vehicles, it is no longer fixed by anything and, after dismantling the wheel, can be easily removed by hand. In the case of fastening, in addition to wheel bolts, it is also difficult to remove the drum with guide screws. Often, after unscrewing these screws, the drum cannot be removed from the centering landing belt of the axle shaft. Especially if it has never been removed after the factory assembly and, as they say, has become attached.
You should, of course, try to tighten the removed guide screws into the special threaded holes, acting with them as a puller. However, often this only leads to stripping of the thread, and the drum does not move. Then proceed to the next operation. Having securely fixed the wheels standing on the ground with linings, they start the engine, turn on a lower gear and, while rotating the drum at a low speed, sharply press the brake pedal. If the hydraulic drive does not work, brake sharply with a parking brake. As a rule, under such dynamic loading, the axle shaft rotates in the drum hole, after which it is no longer difficult to remove the drum.
After removing the drum, loosen the parking brake cable and remove its tip from the lever. Having removed the cotter pin from the hole of the finger, remove the lever (on some car models, the removal of this lever is not necessary). Next, the brake pads are released from the support shield by removing the guide springs, and proceed to the removal of the pads themselves. A common mistake in this case is an attempt to remove, first of all, the upper spring that tightens the pads. This is very difficult to do, especially with bare hands or with just a screwdriver. It is much easier to first remove the lower, weaker spring, then extend the lower ends of the pads from the grooves of the support and, shifting the pads a little towards yourself and acting with them like long levers, stretch the upper spring and remove the parking brake expansion bar, and then the pads themselves from the support slots in the pistons of the brake cylinder. It is very important not to damage the rubber protective caps of the cylinder.
Now you can disassemble the wheel brake cylinder. Here I would like to give some advice. If you do not feel sorry for the old brake fluid, you can immediately remove the protective caps and, using a punch made of soft material, press the pistons complete with automatic clearance adjustment parts out of the cylinder. The liquid will then spill onto the ground. If the liquid is worth saving, it is better to disconnect the tube from the wheel cylinder and plug it with a wooden plug. After that, the cylinder is removed from the support shield and it is continued to be disassembled on a workbench. Using a screwdriver, turn the piston, unscrew the stop screw from it and remove the cuff with the support cup and crackers. Then separate the thrust ring and the screw.
After disassembly, all parts are wiped and carefully inspect the surface of the cylinder and cuffs. The cylinder mirror should be completely clean and smooth, without scratches and roughness, and there should not be even the slightest bumps on the outer surface of the cuffs.
Reference. Small defects on the cylinder mirror can be eliminated by grinding, taking into account that an increase in its diameter is highly undesirable. The cuffs should be replaced with new ones, even if the old ones do not have visible wear. It is also necessary to check whether the protective rubber caps of the cylinder are damaged and, if necessary, replace them with new ones.
Before assembling, it is necessary to generously lubricate all parts with brake fluid and perform the above operations in reverse order. After assembly, check the movement of the pistons in the cylinder by squeezing the pistons with your hands. They should move without jerking and jamming.
The general assembly of the brake mechanism is carried out in the reverse order indicated.
The last thing to do is to fill the brake actuator with fluid and remove air from it. To do this, fill the reserve tank with 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, a rubber hose is put on the fitting head, and its free end is lowered into a transparent vessel partially filled with brake fluid. Sharply pressing the brake pedal 3-5 times with an interval of 2-3 s, unscrew the fitting half a turn, continuing to press the pedal (it takes two people). Air with brake fluid will come out of the tube. Further, lowering the pedals, wrap the fitting. Repeat the entire operation until no more air bubbles come out of the tube, the pedal becomes "tough". The same operations are repeated for other wheels of the car.
It should be noted that pumping the system can be done alone. To do this, you must perform all of the specified preparatory operations. However, after unscrewing the fitting, you need to get behind the wheel yourself and press the brake pedal 3-4 times. Then, without removing the tube and without removing it from the liquid in the vessel, screw the fitting to failure. Then get behind the wheel again and check "rigidity" pedals. If its course is at least slightly reduced, proceed to the same operations in turn with the other wheels of the car. When pumping the last brake cylinder, the pedal should be as hard as possible, and its travel should be more than ½ of its full travel. When removing air, it is absolutely necessary to monitor the decrease in the liquid level in the reserve tank and constantly top it up. Of course, the described operations for pumping the brake drive alone will take more time than together. However, then you will once again be convinced that you can cope with this type of repair completely on your own.
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