The foundation of all brake functions is still the hydraulic system. In order to fully understand the highly-technical systems built into Subaru brakes, you must first understand the operation of the basic hydraulic components.

Over the last decade or so, many safety features have been incorporated into Subaru vehicles. These braking, stability, vehicle dynamics and control advances all have one thing in common: They are possible only because of the hydraulic braking system.

The electronically-controlled antilock braking system (ABS) was the first add-on to the basic hydraulic brakes. It offered the ability to monitor wheel speeds, through the use of sensors, to analyze and correct any braking deficiencies. ABS also allowed the creation of other safety systems, such as Vehicle Dynamic Control (VDC) to maintain vehicle stability and help avoid rollover.

Underlying these technical advances is the good old hydraulic braking system. Understanding the basic hydraulic components is essential to an overall working knowledge of the complete braking system. Here’s a look at the components of Subaru hydraulic brakes systems and how they function.

The Hydraulics

Subaru vehicles use a dual-diagonal brake system that employs a master cylinder to feed a crisscross hydraulic circuit consisting of a primary circuit and a secondary circuit.
Braking force is transmitted to the right front and left rear brakes by the primary system while the left front and right rear brakes are fed by the secondary system. This style of system provides the safety of separate circuits and creates balanced braking in the event of a failure in one of the two circuits.

Master Cylinder

The master cylinder is divided into two chambers: the primary chamber, which supplies hydraulic pressure to the right front and left rear brakes, and the secondary chamber, which supplies the left front and right rear brakes. If one of the two independent circuits should fail, the other can still provide stopping power for the vehicle.

On 1997 and earlier models, a large, rounded reservoir topped the master cylinder. It’s easily identified by the large yellow cap. This older style also uses the double-chamber design for primary and secondary braking.

Brake Fluid Level Indicator

Both styles of master cylinders include a brake fluid level indicator to warn the driver of a dangerously low fluid level. A magnet is encased in a float in the brake fluid reservoir. If the fluid level lowers to a point approximately 1.8 inches (30 mm) below the MAX level line, the magnet activates a reed switch that illuminates a warning light.

Brake Booster

The brake booster is connected to the master cylinder. It provides vacuum assist to the brake pedal. The booster is a tandem-type that uses two diaphragms to convert mechanical brake pedal pressure into hydraulic pressure.

Manifold vacuum provides the negative pressure to one of the diaphragms inside the booster that is connected to the brake pedal linkage. Atmospheric pressure then assists in pedal application. A check valve in the vacuum line traps vacuum in the booster. This ensures booster operation even when the manifold vacuum is low.

Note: the brake booster is not serviceable and must be replaced as a unit. The check valve may be replaced separately.

Proportioning Valve (Non-ABS Models)

The proportioning valve reduces the possibility of rear wheel lockup. It does so by controlling the brake fluid pressure available to the rear wheels.

When the pressure in the master cylinder reaches a predetermined point, called the split point, the proportioning valve limits the pressure between the master cylinder and the rear wheel brake cylinders. If either the primary or secondary brake hydraulic circuit fails, the proportioning valve will not control pressure to the rear wheels. The pressure in the operating circuit will remain equal to the pressure in the master cylinder.

Note: Split points may vary by model and year. Always refer to the correct service manual for the vehicle you are servicing.
Electronic Brake Distribution (ABS Models)

On models equipped with ABS, the electronic brake distribution (EBD) valve controls the brake fluid pressure to the rear wheels, eliminating the need for a proportioning valve.

The system optimizes the brake force distribution to the front and rear wheels according to changes in loading conditions or center of gravity during deceleration.

The ABS control module calculates the optimum brake force distribution from the difference in speed between the front and rear wheels and adjusts the pressure through the ABS hydraulic control unit (HCU).

Note: The warning light for EBD failure is the same light that signals a problem with brake system or low hydraulic fluid level.

EBD may be found on these models equipped with ABS:

• 2003: Forester
• 2004: Forester, Impreza & Outback Sport
• 2005 and later: All Models

Hill Holder

Hill Holder is a unique system found on many Subaru vehicles equipped with manual transmissions. It prevents the vehicle from rolling backwards during a stop on an uphill grade.
When the driver comes to a stop and fully presses both the clutch and brake pedals, the Hill Holder retains the pressure in the primary brake circuit after the brake pedal is released.
The pressure is released when the clutch is released at the point when the driver presses the accelerator pedal.

The control unit for the Hill Holder system is the pressure hold valve (PHV). It is connected to the primary hydraulic brake circuit and linked mechanically, via a cable, to the clutch pedal assembly.

When the vehicle stops on an uphill grade of three degrees or more, a push rod inside the PHV retracts when the clutch is pressed. This permits a check ball inside the PHV to roll backwards, thereby sealing the pressure in the primary circuit.

When the brake pedal is released, the pressure trapped in the primary circuit by the ball holds the vehicle stationary. When the clutch pedal is engaged, the push rod extends to unseat the ball and release the hydraulic pressure.

The Hill Holder is only activated when stopping or parking on an uphill incline. However, the effects of system activation may be felt occasionally on older models when the vehicle is in reverse and backing up.

This condition is not abnormal.

For Hill Holder information for the model you are servicing, refer to the specific service manual or online at the Subaru Tech Information System website at http://techinfo.subaru.com. Replacement parts are available through your Subaru N.E.W. Horizons Dealer.

Note: The pressure hold valve (PHV) is not serviceable and must be replaced as a unit.

Hill Holder may be found in the following models:

• 1990-1999 Legacy
• 1993-1996 Impreza
• 1995-1999 Outback
• 1995-1997 Outback Sport
• 2003-2007 Forester