They use hydraulic force to operate brakes, because it is simple and dependable. When things go wrong, we need a bit of understanding to find solutions.
Basic brake hydraulic operation
Automotive braking systems use hydraulic force to transfer motion. The brakes work because we cannot compress brake fluid. We apply pressure to a piston in a cylinder and this reduces the fluid capacity. Brake fluid under pressure now moves to other cylinders connected to the system. Pressure applied to the first piston transfers to the second. This force moves the second piston. This is the basis of most hydraulic brake operation.
An example of brake system hydraulics is the driver pressing the brake pedal to stop. The brake pedal pushes pistons in the master cylinder creating hydraulic pressure. This pressure forces brake fluid into the system. Steel lines and hoses connect the master cylinder to wheel cylinders and brake calipers.
Brake fluid, under pressure, flows into the cylinders and calipers at each wheel. This pressure causes the pistons in each to move, applying the brakes. When we release the brake pedal, the pistons retract and fluid returns to the brake master-cylinder reservoir.
Wear and additional fluid
The brake pads and shoes accomplish braking by creating friction. The hydraulic system simply transfers the motion needed to apply them. Friction causes the brake pads and shoes to wear. Wear creates space and this take additional brake fluid into the system to make up for it. The fluid comes from the master cylinder reservoir and moves to the calipers and wheel cylinders. Normal brake pad and shoe wear causes the fluid level to drop in the master cylinder over time.
Even with such a dependable system, things still go wrong. They make brake fluid from alcohol, to absorb moisture. In time the fluid becomes saturated and can no longer hold the contamination. Moisture in the system attacks the metal surfaces of the brake cylinders.
Corrosion and leakage
Corrosion damages the sealing surfaces of cylinders and causes them to leak. To hold pressure, they design each cylinder with seals. Fluid can run passed the internal seals if corrosion damages the cylinder surface. When this happens, we must replace the cylinder.
External seals are in place to keep debris out of the cylinder. This is different from the internal seals, which hold pressure. They do not design external seals to hold fluid. If we find fluid behind the outer wheel cylinder seal, the internal seal has failed.
Leaking wheel cylinders can also allow air to enter the system when we release the brakes. This often occurs after replacing brake shoes and not replacing the wheel cylinders. Air is compressible and results in a spongy brake pedal. The brake pedal moves farther, before applying the brakes. Instead of moving the pistons in the cylinders, we waste hydraulic pressure compressing air in the system. A leaking brake caliper can cause the same type problem.
Bleeding the brakes
We must purge any air that enters the brake system. We call this process bleeding the brakes. With pressure applied, a screw is opened at a high point in the system. The air rises to the highest point and is forced out through the opening. We then tighten the screw and release the pressure.
Because of the huge variation in braking systems, several different methods are used. The order in which the wheels are bled and the means of applying pressure vary greatly. Some systems require the right rear wheel to be bled first. Other systems may start with the left front and so on. Bleeding the system out of sequence will not remove the air and may cause additional air to enter.
Cleanliness of the brake system is imperative. Brake fluid becomes contaminated over time and should be replaced. Debris comes from wearing components and sludge builds up over time. Because these materials are heavier than the brake fluid, they collect in the calipers and wheel cylinders.
A common brake problem
The old practice of pushing brake pistons in, when replacing brake pads can cause many problems. Modern brake systems use several delicate and expensive components. Anti-lock brake system or ABS modulators can be ruined by the back-flush of debris caused when caliper pistons are pushed in.
A much better practice is thoroughly flushing old fluid from the system before any brake work is done. The bleeder screw should also be opened when pushing caliper pistons in, to expel debris. Close the bleeder screw once the piston is all the way in. Many shops do not follow this simple procedure and a great many problems are caused as a result.
Diagnosing brake pulls and brake lock up
If fluid is not replaced or when a brake system becomes contaminated problems will develop. Common problems are a pull in the steering when braking and after driving, wheels that lock up. When brakes are applied, pistons move out. When the brakes are released, the pistons must move back into their previous position. Corroded cylinders may cause pistons to stick.
When a piston sticks out, the brake will drag or lock up. This causes a great deal of heat and can quickly destroy the brakes. Material from deteriorating brake hoses also causes brake cylinders to stick.
Pistons can also seize and fail to move when brakes are applied. This will cause a pull on braking. Ironically the pull is normally toward the side that is working. The seized caliper does not apply and the good caliper does. Because the working wheel stops, the pull is toward the braking wheel.
Brake hoses can cause a similar problem. An inner liner can collapse and act much like a check valve. This can cause a brake to pull or stay applied. Diagnosis is simple. Simply open the bleeder screw on the wheel locked up. If the wheel remains locked, the problem is in the caliper. The wheel releasing means fluid pressure is being retained.
With the wheel again locked, loosen the line above the brake hose. If the brake releases, the problem is further up in the system. The wheel remaining stuck suggest the problem is the brake hose.
Both front or all wheels locking up
This often occurs after master cylinder or brake booster replacement. The return port in the brake master cylinder must remain open. If it is blocked, fluid cannot return to the reservoir. When the fluid heats up and expands the brakes will be applied, causing lock up.
A simple tool is used to check the adjustment on the brake push rod. A sliding center pin is adjusted to touch the push rod tip. The tool is then flipped over and used to check the master cylinder. The push rod is adjusted until it matches the master cylinder.
Brake fluid contaminated with petroleum, such as power steering fluid, will cause a similar problem. Oil causes the seals in the brake master cylinder to swell and block the return port. If petroleum enters a brake system, every piece of rubber in the system will generally have to be replaced. Oil attacks the rubber and causes it to swell and deteriorate.
Petroleum cannot be flushed out of a brake system
Replacing only the master cylinder will insure future problems. Oil is lighter than brake fluid and will continue to attack rubber in the system. The lighter oil will also rise through the brake lines and destroy the new master cylinder.
Hydraulic brake repair is not complicated if done correctly. A few simple points should help keep your brakes working properly for years.
Only add brake fluid from a new and unopened container
Brake fluid should be replaced every two to three years
Thoroughly flush the brake system BEFORE starting any brake repair
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The old practice of pushing brake pistons in, when replacing brake pads can cause many problems. Modern brake systems use several delicate and expensive components. 
Only add brake fluid from a new and unopened container