Well, first of all, it's not that sudden. Most likely, you simply did not notice the leakage starting earlier. Secondly, let's look at the reasons for this type of brake failure, which is unfortunately typical for older vehicles.
Let's recall the basic design principles of modern hydraulic brake drives. 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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Fluid pressure in the lines and wheel brake cylinders during heavy braking can reach 10–12 MPa (100–120 kg/cm²). Therefore, sealing all lines, and especially the moving parts of the drive system, is a critical design requirement. And while fixed pipe connections are relatively easy to seal, moving parts – the pistons of the master brake cylinder, wheel cylinders and booster – are more difficult to seal.
Today, global automotive manufacturers recognize two types of rubber cuff seals: solid, cup-shaped, without a central hole, and rubber washers with a convex outer surface. The first ones are rarely used today, the second ones are more widespread, 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 seal – the cuff – is worn out. It is more of a dust cover that protects the piston from dirt on the outside. The pistons' primary sealing element is the piston seals. Before braking, the pistons are stationary, and the seals do not contact their end surfaces, as they are held in place by spacer rings that rest 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 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 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 fluid 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, 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 fluid pressure, and the operation of its cuff is no different from that described. 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 between braking applications 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 master brake cylinder (air, unfortunately, will not be removed from the wheel cylinders on its own - the system must be bled).
So what should you do if your brakes are leaking? Let's start with the most common: repairing the wheel brakes. Rear drum brake cylinders leak more often than front disc brakes (for disc brakes, pistons tend to seize up, not seals leak). 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 bolts or nuts, 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 removing the wheel. 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 the special threaded holes, using them as a puller. However, this often only leads to stripping of the threads, and the drum does not move. Then they resort to the following operation. Having securely fixed the wheels on the ground with chocks, 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 using 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. After removing 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 brake 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 grooves and, moving the pads slightly towards you 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 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 and automatic clearance adjustment components out of the cylinder. The liquid will spill onto the ground. If the fluid 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, keeping in mind 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 reservoir with fresh brake fluid to the "MAX" mark. Then, begin bleeding the wheel cylinder of the outermost wheel. To do this, place a rubber hose on the head of the fitting, and lower its free end into a transparent container partially filled with brake fluid. Sharply pressing the brake pedal 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 entire procedure until no more air bubbles come out of the tube and the pedal becomes stiff. Repeat these steps for the other wheels of the vehicle.
It should be noted that pumping the system can be done alone. To do this, it is necessary to complete all the specified preparatory operations. However, after loosening the nipple, you should get behind the wheel and press the brake pedal 3-4 times. Then, without removing the tube or removing it from the fluid in the container, tighten the nipple until it stops. Then get back behind the wheel and check the pedal's firmness. If its travel has decreased even slightly, repeat the same procedure on the other wheels of the car. 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 fluid level in the reserve tank and constantly top it up. Of course, the described operations for bleeding the brake drive will take more time if done alone than by two people. However, then you will once again be convinced that you can handle this type of repair quite independently.