Things are bad. The trip is out of the question. Repairs are needed. But why did this happen? And why so suddenly?
Well, first of all, it's not that sudden. Most likely, you just didn't notice the leak before. And secondly, let's figure out the reasons for such a brake drive failure, unfortunately, typical for cars of a respectable age.
Let's recall the basics of the design of a modern hydraulic brake drive. 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 ensures the operation of the right front and left rear wheels, the other - the left front and right rear. Such a diagonal circuit division scheme is widely used today in passenger cars of leading global companies. It significantly increases traffic safety compared to previously used schemes.
Note: The fluid pressure in the pipelines and wheel brake cylinders during intensive braking can reach 10-12 MPa (100-120 kg/cm²). Therefore, sealing of all pipelines, and especially moving parts of the drive, is the most important requirement for the design. And if fixed pipeline connections are relatively easy to seal, then moving parts - pistons of the main brake cylinder, wheel cylinders and booster - are more difficult to seal.
The world practice of car manufacturers today knows two types of rubber seals of cuffs: solid, cup-shaped, without a central hole and in the form of rubber washers with a convex outer surface. The first are rarely used today, the second are the most common, as cheaper and more technologically advanced.
Let us consider the design and operating principle of such a seal using the example of a typical design of a tandem brake master cylinder. Its body contains two movable pistons, creating two independent circuits of the brake drive. The tightness of the rear part of the piston is provided by a rubber cuff. However, this cuff will not prevent the fluid from leaking out if the main seal - the cuff - is worn out. It is rather a dust cover that protects the piston from dirt from the outside. The main piston seal is the cuff. Before braking, the pistons are stationary, and the cuffs do not touch their end surfaces, since they are held by spacer rings resting against the mounting bolts. In this position of the pistons, the cavities of the cylinder 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 cross-section. Their outer diameter in a free state slightly exceeds the inner diameter of the brake cylinder. Therefore, if the cuff is not subjected to brake fluid pressure, then only its middle outer belt contacts the cylinder mirror. That is why, with the slightest wear of the outer side of the cuff - even if it is just one single mark - the fluid will begin to flow out. And this, mind you, when the car is parked.
When the brake pedal is pressed, the piston moves forward and comes into contact with the end surface of the cuff. The reliability of the contact is ensured by a spring. From this moment on, the communication of the internal cavity with the reserve tank is interrupted, 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 with its entire external surface.
The second, floating, piston of the main brake cylinder moves under the pressure of the liquid, and the work of its cuff is no different from that described. The cuffs of the wheel brake cylinders work in exactly 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.
Note. 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 eliminating self-jamming of the brake mechanisms. In addition, free circulation of fluid during warming up and subsequent cooling of the system reduces the likelihood of clogging the cylinders with dirt, and also facilitates the spontaneous removal of air bubbles from the main brake cylinder (unfortunately, air will not be removed from the wheel cylinders by itself - the system must be bled).
So what to do if the brakes are leaking? Let's start with the most common - repairing the wheel brake mechanisms. The rear drum brake cylinders leak more often than the front disc brakes (for disc types, pistons tend to jam, rather than seals leaking). We offer a sequence of actions that any driver can perform and do not require any special equipment.
First of all, you need to loosen the wheel mounting bolts or nuts, then lift the car with a jack and place a safety stand under it to remove the wheel. Next, you need to remove the brake drum. On many cars, it is not secured with anything else and can be easily removed manually after the wheel is removed. However, if the drum is secured with guide screws in addition to the wheel bolts, removing it is a bit of a challenge. Often, after unscrewing these screws, the drum cannot be removed from the axle shaft centering seat. Especially if it has never been removed after factory assembly and is said to have become stuck.
Of course, you should try to screw the removed guide screws into special threaded holes, using them as a puller. However, this often only leads to thread stripping, and the drum does not budge. Then resort to the following operation. Having securely fixed the wheels standing on the ground with pads, start the engine, engage a low gear and, while the drum is rotating at a low speed, sharply press the brake pedal. If the hydraulic drive does not work, sharply brake with the parking brake. As a rule, with such a dynamic load, the axle shaft rotates in the drum hole, after which removing the drum is no longer difficult.
After removing the drum, loosen the parking brake cable and remove its tip from the lever. Remove the cotter pin from the pin hole and remove the lever (on some car models, removing this lever is not necessary). Next, release the brake shoes from the support shield by removing the guide springs, and proceed to remove the shoes themselves. A common mistake is to try to remove the upper spring first, which tightens the shoes. 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 push the lower ends of the shoes out of the support grooves and, moving the shoes a little towards yourself and acting with them like long levers, stretch the upper spring and remove the parking brake release bar, and then the shoes themselves from the support slots of the brake cylinder pistons. 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 you some advice. If you do not mind wasting old brake fluid, you can immediately remove the protective caps and, using a soft punch, press the pistons together with the automatic clearance adjustment parts out of the cylinder. The fluid will spill out onto the ground. If you want to save the fluid, it is better to disconnect the tube from the wheel cylinder and plug it with a wooden plug. After that, remove the cylinder from the support shield and continue disassembling it on the workbench. Turn the piston with a screwdriver, unscrew the stop screw from it and remove the cuff with the support cup and crackers. Then disconnect the stop ring and the screw.
After disassembly, all parts are wiped and the surface of the cylinder and cuffs is carefully inspected. The cylinder mirror should be completely clean and smooth, without scratches and roughness, and there should not be even the slightest unevenness on the outer surface of the cuffs.
Note. Minor defects on the cylinder mirror can be eliminated by grinding, taking into account that increasing 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 assembly, 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 or jamming.
The general assembly of the brake mechanism is carried out in the reverse order to that indicated.
The last thing to do is to fill the brake drive with fluid and remove the air from it. To do this, fill the reserve tank with fresh brake fluid to the "MAX" mark. Then proceed to remove air from the wheel cylinder of the outermost wheel. To do this, put a rubber hose on the head of the nipple, and lower its free end into a transparent container partially filled with brake fluid. Sharply pressing the brake pedal 3-5 times at intervals of 2-3 seconds, unscrew the nipple by half a turn, continuing to press the pedal (this requires two people). Air with brake fluid will come out of the tube. Then, lowering the pedals, tighten the nipple. Repeat the entire operation until air bubbles stop coming out of the tube and the pedal becomes "hard". The same operations are repeated for the other wheels of the car.
It should be noted that the system can be bled alone. To do this, you need to perform all the specified preparatory operations. However, after unscrewing the nipple, you need to get behind the wheel yourself and press the brake pedal 3-4 times. Then, without removing the tube and without taking it out of the liquid in the vessel, screw the nipple all the way in. Then get behind the wheel again and check the "rigidity" of the pedal. If its travel has decreased even slightly, proceed to the same operations in turn with the other wheels of the car. When bleeding the last brake cylinder, the pedal should be as rigid as possible, and its travel - 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, alone, the described operations for bleeding the brake drive will take more time than with two people. However, then you will once again see that you can cope with this type of repair quite independently.
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