Modern cars use tandem master cylinders to suit divided or dual line braking systems.
A divided system is safer in the event of partial failure. Fluid loss in one half of the system still leaves the other half able to stop the vehicle, although with an increase in stopping distance.
A wheel’s braking ability depends on the load it’s carrying during braking. So the type of vehicle is a major factor in how its system should be divided.
A front-engined rear-wheel drive car has around 40% of its load on its rear wheels, so its braking system can be divided in a vertical or front-rear split. This puts the front wheels in a different system from the rear wheels. If one half of the system fails - the front or the back - there’s still enough separate braking capability left in the other half, to stop the vehicle.
This doesn’t work well for a front-wheel drive vehicle. A load of about 20% on the rear wheels can’t provide enough braking force to stop the vehicle.
Front-engined, front-wheel-drive vehicles use a braking system split in a diagonal or X. The left hand front brake unit is connected to the right hand rear unit, and the left hand rear, to the right hand front. If one system fails, a 50% braking capability is left in the other system. Dual proportioning valves maintain optimum braking in each system.
A system that partially failed would cause severe braking pull on a vehicle’s suspension. So suspension geometry is usually revised to counter this.
An alternative arrangement for front-engined, rear-wheel-drive vehicles is an L split. The front disc brake units have 4-piston calipers. 2 of the pistons on each front unit connect to the right hand rear, and the other 2 pistons of each unit connect to the left hand rear. As with the X split, if there is failure of either half of the system, it still leaves 50% braking capability.