Master cylinder

In automotive engineering, the master cylinder is a control device that converts force (commonly from a driver's foot) into hydraulic pressure.

For hydraulic brakes or clutches alike, flexible high-pressure hoses or inflexible hard-walled metal tubing may be used; but the flexible variety of tubing is needed for at least a short length adjacent to each wheel, whenever the wheel can move relative to the car's chassis (this is the case on any car with steering and other suspension movements; some drag racers and go-karts have no rear suspension, as the rear axle is welded to the chassis, and some antique cars also have no rear suspension movement).

This is done in a diagonally split hydraulic system i.e. one circuit operates front left and right rear brakes, while the secondary works the other two wheels.

If there is a failure in one of the brake lines or the caliper seal, one of the circuits will still be intact and still be able to stop the vehicle.

With only one circuit working there are significantly longer stopping distances and repairs should be done before driving again.