The brake hydraulic system consists of the master cylinder, disc brake callipers (disc brakes), wheel cylinders (drum brakes), hydraulic lines and hoses, and combination/proportioning valve. When you push on the brake pedal, the force of your leg generates hydraulic pressure in the master cylinder, which then flows through the hydraulic lines and hoses to the wheel cylinders and callipers. The hydraulic force applies pressure through the wheel cylinders and callipers, forcing the shoes against the drums (drum brakes) and the pads against the rotors (disc brakes).
In the early 1960s, cars began using split hydraulic systems and tandem master cylinders. Essentially, this divided the hydraulic system into two separate systems (front and back), ensuring proper hydraulic and braking on one side of the system, if a leak developed on the other side.
In the 1980s, some carmakers began to use diagonally split systems, which took safety a step further. Instead of splitting the system into front and rear, the system was now hydraulically divided into left-rear/right-front and right-rear/left-front. By maintaining one front and one rear brake, the result is more balanced braking when the system develops a leak. By law, all of today's cars are required to use some type of hydraulically split system.
The hydraulic system transmits and multiples force as needed to provide braking action throughout the brake system.
Check the brake fluid level in the master cylinder reservoir regularly, ideally at every oil change. Most cars use semi-transparent reservoirs that have level markings to make fluid monitoring easy. If you need to add fluid, use only the type of fluid recommended in your car's owner's manual. Do not leave the cover off the master cylinder any longer than necessary; DOT 3 and DOT 4 brake fluids attract water. Use extreme care when handling DOT 3 and DOT 4 fluid as it quickly destroys paint if spilled. On cars with disc brakes, it's normal for the fluid level to gradually drop as the brakes wear. This fluid fills up the space left by the disc brake calliper pistons as they move outward with brake wear.
However, if you find that brake fluid needs to be added frequently, there may be a leak in the hydraulic system. Have the system inspected as soon as possible by a qualified service technician. Your owner's manual may specify periodic flushing and filling of the brake hydraulic system, which should not be overlooked.
This is a service best left to professional technicians, as many cars with ABS have specialized brake-bleeding procedures. Braking systems with ABS can also generate extremely high hydraulic pressures, which can be dangerous. Once again, consult a professional service technician if your car needs ABS or hydraulic system service.