Overview
With a basic master cylinder in the braking system, any loss of fluid, say because a component fails, could mean the whole braking system fails.
To reduce this risk, modern vehicles must have at least 2 separate hydraulic systems. That’s why the tandem master cylinder was introduced.
Like 2 single-piston cylinders end-to-end, a tandem cylinder has a primary piston and a secondary piston. Each section of the cylinder has inlet and outlet ports, and compensating ports.
There can be 2 separate reservoirs, or just one but it is divided into separate sections.
When the brake is applied, the primary piston moves, and closes its compensating port. Fluid pressure rises, and acts on the secondary piston. It moves, closing its compensating port. Pressure builds up in this circuit. Both pistons then move, and displace fluid into their separate circuits and apply the brakes.
If there is a failure in the secondary circuit, the primary system continues to operate normally, but with increased travel.
If the primary circuit fails, no pressure is generated to move the secondary piston. So a rod attached to the front of the primary piston will push the secondary piston directly so that it still operates.
A switch can warn of loss of pressure in the front or rear circuits. Or one that warns of low fluid level can be fitted to the reservoir.
The tandem master cylinder just like the single piston master cylinder can have problems with a low-pressure area developing when the piston returns quickly but the fluid lags. The tandem master cylinder overcomes this by using grooves in the side of the primary cup. These grooves allow fluid to flow from the inlet port into the low-pressure area.
Operation
In a similar way to the single brake master cylinder, the tandem master cylinder displaces hydraulic brake fluid the the brake system components when the driver applies pressure to the brake pedal. As with the single master cylinder, the tandem master cylinder reservoir contains the majority of fluid for the brake system. With the single system there was only one operational chamber for both the front and rear braking circuits. With the tandem system there is effectively an operating chamber for the front brakes and a separate operating chamber for the rear brakes, but operated from a common brake pedal.
The main reason for the use of a tandem braking master cylinder is for safety reasons. In the advent of a leak or failure in one circuit the other circuit will still have a braking potential, but with a reduced effeciency.The system utilises two sub-systems.
The tandem master cylinder is used in a split braking system. The type of split systems include:
When the brake pedal is depressed, the push rod connected to the pedal moves the 'primary' piston contained in the master cylinder forward. The primary piston now activates one of the sub-systems. The hydraulic pressure created by the primary piston and spring now moves the secondary piston forward thus activating the other sub-system.
With the forward movement of the pistons the primary cup seal and the secondary cup seal closes off the supply ports from the reservoir and creates an increasing hydraulic pressure build-up. The building pressure is transmitted to the brake stations and the brakes are applied.
As the brake pedal is released, the rearward movement of the pistons the primary cup seal and the secondary cup seal uncover the supply ports to the reservoir and decreases the hydraulic pressure build-up. Thus releasing the brakes.
Electronic Monitoring of System
Some manufacturers fit electronic sensors within the master cylinder reservoirs to monitor the hydraulic brake fluid levels. If the fluid levels drop below the sensor contacts, the driver is alerted by a light coming on in the vehicle's dash.
The more common monitoring device is an electronic switch that monitors the functionality of the sub-systems. While each sub-system is operating correctly, a pistion remains balanced between the the two chambers. If a failure occurs in either sub-system, the with the extra movement of the primary or secondary pistons an electronic switch is activated as a spring-loaded drops down and closes the electrical circuit. The driver is alerted to this malfunction when the brake light on the vehicle's dash is illiminated.
In either of these situations, the driver should have the braking system inspected and repaired as necessary by a suitably qualified automotive technician.
Source: CDX Global & Wikipedia - en.wikipedia.org
