Car Brakes Explained
When you press the brake pedal on your car, several things are going on. The force of your foot on the pedal is multiplied and converted to pressure; that pressure is transmitted through the system, and friction converts kinetic energy into heat. Then the vehicle slows or stops depending on the amount of force you apply. A complex sequence of events leads up to this.
Brake Pedal Function
The brake pedal is a simple class 2 lever attached to the master cylinder piston by a rod. As force is applied to the brake pedal, it is multiplied through leverage. For example, the brake pedal on a 95 Mustang has a ratio of 3 to 1. The force is multiplied by 3, i.e., a force of 100 psi becomes 300 psi at the master cylinder.
The master cylinder converts force into system pressure. The rod attaching the pedal assembly to the master cylinder, moves the master cylinder piston down the bore of the master cylinder. The key thing to remember here is Pascal’s Law. It states that fluids are virtually non-compressible, and any force applied to a closed hydraulic system will be felt equally and instantly throughout the entire system. So, the brake fluid is forced, under pressure, down the bore of the master cylinder by the moving piston into the rest of the system.
Combination Valve Function
The combination valve is the next part of the system, and it allows us to control how much pressure is applied to each wheel, and when it is applied. Without the combination valve, the vehicle would have to be designed with equal weight, and identical brake assemblies, at all four wheels.
The combination valve is made up of three types of parts. First, the pressure differential switch is the part that turns on a brake warning light when part of the system fails. As long as fluid pressure is equal in both sides of the system, the contact is centered in the switch and no light is on. If one side of the system has a pressure loss, the higher pressure on the other side of the system will push the contact and provide a ground for the light to be illuminated. All combination valves have this feature.
The second part is the proportioning valve. This part lowers hydraulic pressure to the rear brakes, improving brake bias and control during heavy braking. While some proportioning valves are found in the combination valve it may also be found in the outlet of the master cylinder, or on the rear suspension instead.
The last part the combination valve may contain is the metering valve. This valve holds off pressure from the front brakes until the rear drum brake shoes overcome return spring tension and make contact with the drum. This prevents front brake lock-up during light pedal application during slippery driving conditions.
The combination valve may contain all or some of these parts, depending on brake system design.
Calipers and Wheel Cfylinders
Calipers and wheel cylinders are the hydraulic output parts of the brake system, and multiply pressure while applying force to pads and shoes.
As the pressure from the force of the brake pedal application enters the caliper it meets up with the second part of Pascal’s Law. The output force is directly proportionate with the ratio of the input piston diameter to output piston diameter. For example, if the diameter of the master cylinder piston is one inch and the diameter of the caliper piston is 2 inches and we assume an input force of 100 psi, then the output will be 200 psi. This is the amount of clamping pressure the pads have on the rotor.
The force of the pads clamping on the rotor, or shoes pressing against the drum, now can convert the kinetic energy of the moving vehicle into heat energy. As the kinetic energy is converted, less and less of it is available for conversion, and the vehicle slows and eventually stops.