Hydraulic Brake Systems

Hydraulic operation of brakes has been the universal design for more than 60 years. The complete hydraulic system consists of master cylinder; steel lines, rubber hoses, various pressure-control valves, and brake apply devices at each wheel.

Master Cylinder

The master cylinder is the start of the brake hydraulic system. It actually is a cylindrical pump. The cylinder is closed at one end, and the flexible pushrod extends from the other end. The pushrod moves a pair of in-line pistons that produce the pumping action. The brake pedal lever moves the pushrod this moves the pistons to draw fluid from a reservoir on top of the master cylinder. Piston action forces the fluid under pressure through outlet ports to the brake lines. All master cylinders for vehicles built since 1967 have two pistons and pumping chambers. Motor vehicle safety standards involve this dual-brake system to provide hydraulic system operation in case one wheel brake assembly loses fluid. Because the brake hydraulic system is closed, all the lines and cylinders are full of fluid at all times. The master cylinder develops system pressure the amount of fluid moved is only in less value.

Split Systems

Modern-day vehicles have split brake systems. The pre-1970s vehicle had a single hydraulic system serving all four wheels. A leak anywhere in the system will result in a complete braking failure. The split system is designed to prevent total system failure. This required the use of a dual-piston master cylinder and the inclusion of various valves. A split system is fed by one piston in the master cylinder and feeds two wheel brakes of the vehicle.

There are two types of split systems: diagonal and front/rear. The diagonal system has one system feeding a front-wheel brake and the rear opposing side wheel brake, that is left front and right rear. The second triangle split is to the other wheel brakes. One side or split feeds the rear-wheel brakes and the other feeds the front wheels. Both of these types have advantages and disadvantages, but each prevents complete system failure from a single leak.

Brake Lines and Hoses

The rigid lines or pipes of a brake hydraulic system are made of steel tubing for system safety. Flexible rubber hoses join the wheel brakes to the rigid 7lines on the vehicle body or frame. The front brakes have a rubber hose at each wheel to allow for steering movement. Rear brakes may have different hoses at each wheel or a single hose connected to a line on the body or frame if the vehicle has a rigid rear axle. Brake lines and hoses contain the high-pressure fluid, and the fluid acts like a solid rod to transfer force to the wheel cylinders and caliper pistons.

Wheel Cylinders and Caliper Pistons

Technically, the wheel cylinders of drum brakes and the caliper pistons of disc brakes are “slave” cylinders because they operate in response to the master cylinder. These hydraulic cylinders at the wheels change hydraulic pressure back into mechanical force to apply the brakes. Most late-model systems with drum brakes have a single, two-piston cylinder at each wheel. Hydraulic pressure enters the cylinder between the two pistons and forces them outward to act on the brake shoes. The shoes move outward, the lining contacts the drums to stop the car. The caliper pistons for disc brakes also act in response to hydraulic pressure that enters a fluid chamber in the caliper. Hydraulic pressure in stationary caliper is applied to one or two pistons on each side of the caliper to force the pads against the rotor. Pressure is applied to a single piston in a movable caliper on the inboard side to force the inboard pad against the rotor. Hydraulic pressure is equal in all directions in a closed chamber. This equal pressure creates a reaction force that moves the outboard side of the caliper inward so that both pads grip the rotor.