Older systems have manual adjusters that need to be turned from time to time as the friction linings wear. Shoe travel is kept as short as possible by an adjuster. In either type, return springs pull the shoes back a short way when the brakes are released. It is simpler but less powerful than the two-leading-shoe system, and is usually restricted to rear brakes. This design allows the two shoes to be forced apart from each other by a single cylinder with a piston in each end. Some drums have twin leading shoes, each with its own hydraulic cylinder others have one leading and one trailing shoe - with the pivot at the front. The rotation of the drum tends to pull the leading shoe firmly against it when it makes contact, improving the braking effect. A leading shoe has the piston at the leading edge relative to the direction in which the drum turns. Has a pivot at one end and a piston at the other. But much more force must be exerted on the brake pedal to apply them. Stops, for example the brakes still work because there is a normal mechanical link between the pedal and the master cylinder. When the pedal is released, the space behind the diaphragm is reopened to the manifold, so the pressure drops and the diaphragm falls back. If the pedal is then held, and pressed no further, the air valve admits no more air from outside, so the pressure on the brakes remains the same. The higher pressure of the outside air forces the diaphragm forward to push on the master-cylinder piston, and thereby assists the braking effort. Pressing the brake pedal closes the valve linking the rear side of the diaphragm to the manifold, and opens a valve that lets in air from outside. When the brakes are off, both sides of the diaphragm are exposed to the vacuum from the manifold. The brake pedal pushes a rod that in turn pushes the master-cylinder piston.īut the brake pedal also works on a set of air valves, and there is a large rubber Such systems apply and release the brakes in rapid succession to stop them locking.Ī direct-acting servo is fitted between the brake pedal and the master cylinder. It closes when heavy braking raises hydraulic pressure to a level that might cause the rear brakes to lock, and prevents any further movement of fluid to them.Īdvanced cars may even have complex anti-lock systems that sense in various ways how the car is decelerating and whether any wheels are locking. Most cars now also have a load-sensitive pressure-limiting Under heavy braking, so much weight may come off the rear wheels that they lock, possibly causing a dangerous skid.įor this reason, the rear brakes are deliberately made less powerful than the front. Sometimes one circuit works the front brakes and one the rear brakes or each circuit works both front brakes and one of the rear brakes or one circuit works all four brakes and the other the front ones only. Most modern cars are fitted with twin hydraulic circuits, with two master cylinders in tandem, in case one should fail. To be exerted by the brakes, in the same way that a long-handledĬan easily lift a heavy object a short distance. The combined surface 'pushing' area of all the slave pistons is much greater than that of the piston in the master cylinder.Ĭonsequently, the master piston has to travel several inches to move the slave pistons the fraction of an inch it takes to apply the brakes. When you push the brake pedal it depresses aĪt each wheel and fills them, forcing pistons out to apply the brakes.ĭistributes itself evenly around the system.
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