The wikepedia article on drums brakes is terrible and misleading. I am surprised how much it has been regurgitated over the internet. Beware wikepedia is not always correct and complete. That being said here is a much better explanation.
Three brake designs are in general use:
There may be others but not as common.
single leading shoe,
twin leading shoe; and
duo-servo.
Each one uses the wedging or self-energizing action of the brake shoe, to assist the lining to grip the rotating drum when the brakes are applied. The twin-leading shoe has an actuator for each brake shoe. The actuator can be mechanical, however a hydraulic actuator is popular on light vehicles. The hydraulic actuator is called the wheel cylinder.
Some brakes have two wheel cylinders, with one piston in each cylinder. When the brakes are applied, hydraulic pressure forces each piston to move outwards, pushing on one end of the brake shoe. The direction of rotation of the drum produces a wedging action on both brake shoes, so they are both called leading shoes.
This system was once popular on front wheels because it is very efficient in the forward direction. This is due to the self-energizing or self-wedging action of the shoes as the drum rotates. Its main disadvantage is that it is only about 30% as efficient in reverse, so it is usually combined with a single leading shoe arrangement on the rear to provide a balanced system.
The single leading shoe system uses a single wheel cylinder with two pistons. Both shoes ride against a fixed pin on the opposite side of the piston. When the brakes are applied, both shoes press against the brake drum. One shoe is called leading shoe, the other is called trailing. The leading shoe tends to be self-energized, while the trailing shoe tends to be forced off the drum.
This arrangement is common on rear wheels as they work equally well in forward and reverse, so it makes an effective handbrake. They can also have a self-adjusting mechanism. The primary shoe will be the longer one.
The duo-servo design also uses one wheel cylinder with two pistons. It is a high energy brake, that is, it exerts large self-energizing forces. The lower ends of the shoes are linked but aren’t firmly anchored to the backing plate. This lets the complete shoe assembly float, within limits.
When the brakes are applied, both shoes are carried around by the drum, until the secondary shoe contacts the anchor pin. The self-energizing force of the primary shoe and its wheel cylinder application force is now transferred to the secondary shoe through the lower linkage. The secondary shoe is now being force into the drum by the force from primary shoe trying to rotate through the linkage and its own wedging action. The primary shoe has the shorter lining and is always fitted ahead of the wheel cylinder in terms of drum rotation. The primary shoe has less force applied to it, to keep pressure consistent on the friction material the pad length is shortened. Reducing the area of the primary pad promotes even wear between the primary and secondary. It’s most important that the shoes are fitted correctly, since it’s the secondary shoe that does most of the work. The linings may also have different frictional values. The colors of the retraction springs indicate different spring strengths. This design is common on rear wheels and it works in both directions. The shorter shoe does lessen the self-energizing effect when backing up.
For those wondering Ford has been using the duo-servo design in pickup for quite some time. In other words your Ford pickup with an idi has duo-servos in the back.
Two experiments one can do to see self-energizing effect.
1. Apply park brake and back up. Now try to go forward. Note is much easier to backup due the drum brakes self-energize more in a forward direction.
2. Hook up a boat to a 4 wheel drum vehicle, duo-servo type, and back down a steep boat ramp. One will only do this once.
Onto the residual valve.
All drum brakes should have a residual valve. The valve helps keep the shoes closer to the drum. The pressure works against the springs on drum brakes. A residual valve may be in the master cylinder, combination valve or any device further down the line. A drum system without residual valves will have a mushy pedal. On disks as long as the reservoir is above the caliper the residual valve is not needed. The difference in height will provide enough pressure on the system. For drums use a 10lb residual valve. For disks if the reservoir is lower use a 2 lb.
