The brake pedal operates a hydraulic system that is used for 2 reasons. First, fluid under pressure can be carried to all parts of the vehicle by small hoses or metal lines without taking up a lot of room or causing routing problems. Second, the hydraulic fluid offers a great mechanical advantage; little foot pressure is required on the pedal, but a great deal of pressure is generated at the wheels.
The brake pedal is linked to a piston in the brake master cylinder, which is filled with hydraulic brake fluid. The master cylinder consists of a cylinder, containing a small piston and a fluid reservoir.
Modern master cylinders are actually 2 separate cylinders. The 2 cylinders are actually separated, allowing for emergency stopping power should one part of the system fail. The braking force is applied to one rear wheel and one front wheel in diagonal pattern. This system is known as the dual diagonal system.
The entire hydraulic system from the master cylinder to the wheels is full of hydraulic brake fluid. When the brake pedal is depressed, the pistons in the master cylinder are forced to move, exerting tremendous force on the fluid in the lines. The fluid has nowhere to go and forces the wheel cylinder piston (drum brakes) or caliper pistons (disc brakes) to exert pressure on the brake shoes or pads. The resulting friction between the brake shoe and wheel drum or the brake pad and disc slows the vehicle and eventually stops it.
Also attached to the brake pedal is a switch which lights the brake lights as the pedal is depressed. The lights stay ON until the brake pedal is released and returns to its normal position.
Each wheel cylinder in a drum brake system contains 2 pistons, one at either end, which push outward in opposite directions. In disc brake systems, the wheel cylinders are part of the caliper; there can be as many as 4 or as few as 1. Whether disc or drum type, all pistons use some type of rubber seal to prevent leakage around the piston and a rubber dust boot seals the outer ends of the wheel cylinders against dirt and moisture.
When the brake pedal is released, a spring pushes the master cylinder pistons back to their normal position. Check valves in the master cylinder piston allow fluid to flow toward the wheel cylinders or calipers as the piston returns. As the brake shoe return springs pull the brake shoes back to the released position, excess fluid returns to the master cylinder through compensating ports, which have been uncovered as the pistons move back. Any fluid that has leaked from the system will also be replaced through the compensating ports.
All brake systems use a switch to activate a light, warning of brake failure. The switch is located in a valve mounted near the master cylinder. A piston in the valve receives pressure on each end from the front and rear brake circuits. When the pressures are balanced, the piston remains stationary but when one circuit has a leak, greater pressure during the application of the brakes will force the piston to one side or the other, closing the switch and activating the warning light.
Disc brake systems also have a metering valve to prevent the front disc brakes from engaging before the rear brakes have contacted the drums. This ensures that the front brakes will not normally be used alone to stop the vehicle. A proportioning valve is also used to limit pressure to the rear brakes to prevent rear wheel lock-up during hard braking.
Drum brakes use two brake shoes mounted on a stationary backing plate. These shoes are positioned inside a circular cast iron drum which rotates with the wheel assembly. The shoes are held in place by springs; this allows them to slide toward the drums (when they are applied) while keeping the linings and drums in alignment. The shoes are actuated by a wheel cylinder which is usually mounted at the top of the backing plate. When the brakes are applied, hydraulic pressure forces the wheel cylinder's two actuating links outward. Since these links bear directly against the top of the brake shoes, the tops of the shoes are forced outward against the inner side of the drum. This action forces the bottoms of the two shoes to contact the brake drum by rotating the entire assembly slightly (known as servo action). When pressure within the wheel cylinder is relieved, return springs pull the shoes back away from the drum.
Most modern drum brakes are designed to self-adjust during application when the vehicle is moving in reverse. This motion causes both shoes to rotate very slightly with the drum, rocking an adjusting lever. The self-adjusters are only intended to compensate for normal wear. Although the adjustment is "automatic", there is a definite method to actuate the self-adjuster, which is done during normal driving. Driving the vehicle in reverse and applying the brakes usually activates the automatic adjusters. If the brake pedal was low, you should be able to feel an increase in the height of the brake pedal.
Instead of the traditional expanding brakes that press outward against a circular drum, disc brake systems utilize a cast iron disc with brake pads positioned on either side of it. Braking effect is achieved in a manner similar to the way you would squeeze a spinning disc between your fingers. The disc (rotor) is a one-piece casting with cooling fins between the two braking surfaces. This enables air to circulate between the braking surfaces making them less sensitive to heat buildup and more resistant to fade. Dirt and water do not affect braking action since contaminants are thrown off by the centrifugal action of the rotor or scraped off by the pads. Also, the equal clamping action of the two brake pads tends to ensure uniform, straight-line stops. All disc brakes are inherently self-adjusting.There are three general types of disc brake:
- A fixed caliper, 4-piston type.
- A floating caliper, single piston type.
- A sliding caliper, single piston type.
The fixed caliper design uses two pistons mounted on either side of the rotor (in each side of the caliper). The caliper is mounted rigidly and does not move.
The sliding and floating designs are quite similar and often considered as one. The pad on the inside of the rotor is moved into contact with the rotor by hydraulic force. The caliper, which is not held in a fixed position, moves slightly, bringing the outside pad into contact with the rotor. There are various methods of attaching floating calipers; some pivot at the bottom or top and some slide on mounting bolts.
POWER BRAKE BOOSTERS
A vacuum diaphragm is located behind the master cylinder and assists the driver in applying the brakes, reducing both the effort and travel he must put into moving the brake pedal.
The vacuum diaphragm housing is connected to the intake manifold by a vacuum hose. A check valve at the point where the hose enters the diaphragm housing ensures that during periods of low manifold vacuum brake assist vacuum will not be lost.
Depressing the brake pedal closes off the vacuum source and allows atmospheric pressure to enter on one side of the diaphragm. This causes the master cylinder pistons to move and apply the brakes. When the brake pedal is released, vacuum is applied to both sides of the diaphragm and return springs return the diaphragm and master cylinder pistons to the released position. If the vacuum fails, the brake pedal rod will butt against the end of the master cylinder actuating rod and direct mechanical application will occur as the pedal is depressed.
Subaru uses a dual hydraulic system, with the brakes connected diagonally. In other words, the right front and left rear brakes are on the same hydraulic line and the left front and right rear are on the other line. This has the added advantage of front disc emergency braking, should either of the hydraulic systems fail. The diagonal rear brake serves to counteract the sway from single front disc braking.
A leading/trailing drum brake is used for the rear brakes, with disc brakes for the front. All Subarus are equipped with a brake warning light, which is activated when a defect in the brake system occurs.
The hydraulic system is composed of the master cylinder and brake booster, the brake lines, the brake pressure differential valve(s), the wheel cylinders (drum brakes) and calipers (disc brakes).
The master cylinder serves as a brake fluid reservoir and (along with the booster) as a hydraulic pump. Brake fluid is stored in the two sections of the master cylinder. Each section corresponds to each part of the dual braking system. This tandem master cylinder is required by Federal law as a safety device.
When the brake pedal is depressed, it moves a piston mounted in the bottom of the master cylinder. The movement of this piston creates hydraulic pressure in the master cylinder. This pressure is carried to the wheel cylinders or the calipers by brake lines, passing through the pressure differential or proportioning valve.
When the hydraulic pressure reaches the wheels, after the pedal has been depressed, it enters the wheel cylinders or calipers. Here it comes into contact with a piston(s). The hydraulic pressure causes the piston(s) to move, which moves the brake shoes or pads (disc brakes), causing them to contact the drums or rotors (disc brakes). Friction between the brake shoes and the drums causes the vehicle to slow. There is a relationship between the amount of pressure that is applied to the brake peal and the amount of force which moves the brake shoes against the drums. Therefore, the harder the brake pedal is depressed, the quicker the vehicle will stop.
Since the hydraulic system is one which operates on fluids, air is a natural enemy of the brake system. Air in the hydraulic system retards the passage of hydraulic pressure from the master cylinder to the wheels. Anytime a hydraulic component below the master cylinder is opened or removed, the system must be bled of air to ensure proper operation. Air trapped in the hydraulic system can also cause the brake warning light to turn ON , even though the system has not failed. This is especially true after repairs have been performed on the system.
HILL HOLDER SYSTEM
See Figure 1
A feature unique to Subaru, the hill holder is a system designed to engage a single brake channel when a manual transaxle vehicle is stopped on an uphill. The system, in effect, holds the vehicle, to enable ease of starting on an uphill.
The system consists of the basic brake system components and the addition of the Pressure Hold Valve (PHV). The pressure hold valve is connected to one of the service brake pipes. When the clutch pedal is depressed on an uphill, the pushrod in the PHV is pushed in and/or pulled out by the camshaft that is interlinked with the clutch pedal to change the clearance between the PHV ball and the seal. This opens or closes the hydraulic system to the brakes.
The operation of the system is fairly simple; when the car is stopped on an uphill, and the clutch is pressed along with the brake pedal (as in any normal stop) the cam mechanism in the PHV moves the ball valve. This, in effect, applies one side of the brake system and your foot can be taken off of the brake pedal and moved to the gas pedal. As you begin to move the vehicle and release the clutch, the brake system is then also released and you can begin moving ahead. This all has the same effect as stopping on a hill and putting the parking brake on, to make taking off easier.
The hill holder feature is standard on all manual shift models of Subaru from 1985-92, except for the Justy.