Well, first of all, it's not that sudden. Most likely, you simply did not notice the leak starting before. And secondly, let's look into the reasons for such a failure of the brake drive, which, unfortunately, is typical for older cars.
Let's recall the basic 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 provides operation of the right front and left rear wheels, the other - the left front and right rear. This diagonal circuit separation scheme is now widely used in passenger cars from leading global companies. It significantly improves traffic safety compared to previously used schemes.
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The fluid pressure in the pipelines and wheel brake cylinders during intensive braking can reach 10-12 MPa (100-120 kg/cm²). Therefore, the sealing of all pipelines, and especially the 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 - the 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 for cuffs: solid, cup-shaped, without a central hole and in the form of rubber washers with a convex outer surface. The first ones are rarely used today, the second ones are the most common, as they are cheaper and more technologically advanced.
We will consider the design and operating principle of such a seal using the example of a typical design of a tandem-type master brake cylinder. Its body contains two movable pistons, creating two independent brake drive circuits. The rear part of the piston is sealed with a rubber cuff. However, this cuff will not prevent fluid from leaking out if the main cuff seal is worn out. It is more of a dust cover that protects the piston from dirt on the outside. Main piston seal cuff. Before braking begins, 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. In 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 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 comes into contact with the cylinder mirror. This is why, at the slightest wear on the outer side of the cuff - even if it is just one single mark - the liquid will begin to leak out. And this, mind you, is 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 between the internal cavity and the reserve tank stops, and the pressure in the cylinder and in the pipelines begins to increase. Under the action of this liquid pressure, the cuff expands in the radial direction and is securely pressed against the cylinder mirror with its entire outer surface.
The second, floating piston of the main brake cylinder moves under the pressure of the liquid, and the operation of its cuff is no different from that described. The wheel brake cylinder cuffs 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, braking begins smoothly, without jerks, which significantly improves the comfort and safety of the vehicle.
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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 of 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 on its own - the system needs to be bled).
So what to do if the brakes are leaking? Let's start with the most common - repairing the wheel brake mechanisms. Rear drum brake cylinders leak more often than 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 bolts or nuts that secure the wheels, and then, after raising the car with a jack and placing a safety stand underneath it, remove the wheel. Next, you need to remove the brake drum. On many cars it is not secured in any other way and can be easily removed manually after the wheel has been removed. In the case of fastening, in addition to wheel bolts, also with guide screws, removing the drum presents a certain difficulty. Often after unscrewing these screws the drum cannot be removed from the centering seat belt of the axle shaft. Especially if it has never been removed after factory assembly and, as they say, has become stuck.
You should, of course, try to screw the removed guide screws into special threaded holes, using them as a puller. However, this often only leads to the threads being stripped, and the drum will not move. Then they 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 low speed, sharply press the brake pedal. If the hydraulic drive does not work, brake sharply with the parking brake. As a rule, under such dynamic loading, 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 end from the lever. Having removed the cotter pin from the pin hole, remove the lever (on some car models, removing this lever is not necessary). Next, release the brake pads from the support shield by removing the guide springs, and proceed to remove the pads themselves. A common mistake is to try to remove the upper spring that holds the pads together first. This is very difficult to do, especially with bare hands or just a screwdriver. It is much easier to first remove the lower, weaker spring, then push the lower ends of the pads out of the support slots and, moving the pads a little towards you and acting with them like long levers, stretch the upper spring and remove the parking brake release bar, and then the pads 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 the following advice. If you don't 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 liquid will spill onto the ground. If the liquid needs to be saved, it is better to disconnect the tube from the wheel cylinder and plug it with a wooden plug. After this, the cylinder is removed from the support shield and its disassembly continues on the workbench. Using a screwdriver, turn the piston, unscrew the stop screw from it and remove the cuff with the support cup and crackers. Then the thrust ring and screw are separated.
After disassembly, all parts are wiped and the surface of the cylinder and cuffs are carefully inspected. The cylinder mirror must be completely clean and smooth, without any scratches or roughness, and the outer surface of the cuffs must not have even the slightest irregularities.
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Minor defects on the cylinder mirror can be eliminated by lapping, 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 show 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 specified operations in reverse order. After assembly, you should 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 you need to do is fill the brake system with fluid and remove any air from it. To do this, fill the reserve tank with fresh brake fluid up 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 fitting, and lower its free end into a transparent container partially filled with brake fluid. Press the brake pedal sharply 3-5 times with an interval of 2-3 seconds, unscrew the nipple by half a turn, continuing to press the pedal (this requires two people). Air and brake fluid will come out of the tube. Next, lowering the pedals, tighten the nipple. Repeat the operation until air bubbles stop coming out of the tube and the pedal becomes "hard". The same operation is repeated for the other wheels of the car.
It should be noted that pumping the system can be done alone. To do this, you need to perform all the specified preparatory operations. However, after unscrewing the nipple, you need to sit behind the wheel 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 back behind the wheel and check the pedal's "rigidity". If its travel has decreased even slightly, proceed to the same operations with the other wheels of the car in turn. When bleeding the last brake cylinder, the pedal should be as hard as possible, and its travel should be more than 1/2 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, if you do the described operations for bleeding the brake drive alone, it will take more time than if you do it together. However, then you will once again be convinced that you can handle this type of repair quite independently.