There are two circuits within every vehicle’s braking system. These circuits are the master cylinder and combination valves. The combination valve consists of three separate devices that will warn the driver of any problems in the brake system. The master cylinder is a hydraulic mechanism that consists of two pistons and two springs that feed the lines or tubes that lead to the braking system (brakes).
Some cars have a dual braking system where the master cylinder will be divided into two systems. If the master cylinder is divided this way, instead of the front and rear brakes working together, they will work separately.
The master cylinder consists of a reservoir that has a primary and secondary cylinder. This is where the brake fluid is stored in the vehicle. Attached to the reservoir is an electrical sensor. This sensor sends a signal to turn on the brake fluid warning light when the brake fluid is low.
When the brake pedal is depressed, the primary piston forces the brake fluid pressure to the four wheels and the secondary piston compresses the fluid. Once the brake fluid pressure builds up enough, the caliper will close, which will allow the vehicle’s motion to come to a stop. If both pistons are working properly, the amount of pressure should be the same in both brake circuits.
If there is a leak in one of the circuits, the pressure will be lost in the primary cylinder and will now depend on the secondary cylinder. However, the secondary cylinder will function normally, but the master cylinder is actually now functioning as if it has only one piston. The brake pedal will now have to be depressed even further in order to stop the vehicle because the braking force has now been reduced.
The combination valve consists of a metering valve, pressure differential valve, and proportioning valve. The metering valve is what signals the drum brakes (rear braking system) to engage until a certain amount of threshold pressure is reached. Once the proper amount of pressure is reached, this valve will release the pressure to the disk brakes (front braking system). The rear brakes need to engage prior to the front to provide the vehicle with the stability to keep the car straight during braking.
The pressure differential valve has a switch in the brake circuits. This valve is exposed to the pressure from both brake circuits. The piston within this valve has a unique shape and must stay centered. If it goes off-center, this means that it has detected a leak within the braking system and will send a signal to the instrument panel of the vehicle. The proportioning valve is what allows the proper amount of pressure to the rear braking system and allows the front braking system to have the greater braking force.
Without this valve, the rear braking system would lock up. The rear brakes do not require much braking force because most of the weight of the vehicle is on the front brakes, which is why the front braking system has the greater force.