The Basics of Hydraulic Brake Systems, Larry Carley, Counterman, February 2001

The master cylinder is the heart of the hydraulic system. It converts the force exerted on the brake pedal into hydraulic pressure that applies the brakes. Depressing the brake pedal moves a push rod in the master cylinder. Mounted on the push rod, are a pair of pistons (primary and secondary) in tandem (one after the other) that exert force against the fluid in the master cylinder bore. This creates pressure, which along with the fluid that's displaced by the pistons is routed through the brake lines to each of the wheel brakes.

Because brake fluid is incompressible, it acts like a liquid linkage between the master cylinder and each of the wheel brakes. Any increase in pressure that is created in the master cylinder is instantly transferred to each of the brakes. So as the pistons in the master cylinder push against the fluid, it displaces fluid through the brake lines. This pushes the disc brake caliper and wheel cylinder pistons outward to apply the brakes.

When the brake pedal is released, the spring-loaded piston assembly in the master cylinder returns to its rest position. The fluid that was displaced by the pistons is pushed back to the master cylinder as the disc brake pads kick out away from the rotors and springs inside the drums retract the brake shoes. The fluid returns to the fluid reservoir through the "compensating ports," which are small openings between the master cylinder bore and fluid reservoir just ahead of each of the pistons.

DUAL MASTER CYLINDER
Master cylinders are divided into two separate hydraulic circuits, with each having its own fluid reservoir and piston. Up until 1967, most vehicles had single piston master cylinders. The danger with that design was that a leak anywhere in the brake system usually caused the brakes to fail! This is not so with today's dual master cylinders. A dual master cylinder provides redundancy by splitting the brakes into two separate hydraulic circuits. If a leak occurs in one brake circuit, it won't affect the other circuit, leaving at least two brakes operational to stop the vehicle.

With most rear-wheel drive (RWD) vehicles, the hydraulic system is divided front-to-rear, with one circuit for the front brakes and a second circuit for the rear brakes.

On front-wheel drive (FWD) cars, the brake system is usually split diagonally. The left front and right rear brakes share one circuit, while the right front and left rear brakes share the other. This arrangement is necessary because the front brakes do about 80- to 90 percent of the braking, and FWD cars typically have a higher proportion of their weight over the front wheels, plus the front wheels do the driving. If a FWD brake system were split front-to-rear and the front brakes failed, the rear brakes alone might not be able to stop the car.

QUICK TAKE-UP MASTER CYLINDER
Some late model vehicles have a special kind of master cylinder with a "quick take-up" valve. This design reduces brake drag for improved fuel economy. The master cylinder has a stepped bore which delivers a larger volume of fluid when the brake pedal is initially depressed. This allows the caliper piston seals to retract the pistons when the brakes are released so the pads don't drag against the rotors.

PROPORTIONING VALVES AND BRAKE BALANCE
To reduce hydraulic pressure to the rear brakes so the rear brakes don't lock up when the brakes are applied, a "proportioning valve" is required. This valve helps compensate for the differences in weight distribution front-to-rear as well as the forward weight shift that occurs when the brakes are applied.

What we're really talking about here is "brake balance" or "brake bias," which is the difference in the amount of hydraulic pressure channeled to the front and rear brakes. The front brakes on most RWD vehicles normally handle about 60- to 70 percent of the brake load. But on FWD cars and minivans as well as RWD and 4WD pickups and SUVs, the percentage handled by the front brakes can be as much as 90 percent of the load. Consequently, the front brakes need a higher percentage of the total hydraulic force that's applied to keep all four brakes properly balanced.

If the front-to-rear brake force isn't balanced correctly by the proportioning valve, the rear brakes will receive too much brake force causing them to lock up and skid when the brakes are applied.

The other reason for using a proportioning valve to reduce hydraulic pressure to the rear brakes has to do with the design of the brakes themselves. When hydraulic pressure is applied to the wheel cylinder inside a drum brake, the shoes are pushed outward against the drum. When the shoes make contact, the rotation of the drum tries to drag them along. But since the shoes are anchored in place, the drum only pulls the shoes up tighter against itself. Because of this, drum brakes that are "self-energizing" require little additional pedal effort once applied.

Disc brakes, on the other hand, are not self-energizing. It takes increased pedal effort to squeeze the pads against the rotor. In a vehicle that has front disc brakes, and drums in the rear, the drums generate increasingly greater amounts of friction with little additional pedal effort while the discs require increased effort just to maintain the same amount of friction. So the proportioning valve splits the applied pressure so each brake receives just the right amount.

Inside the proportioning valve is a spring-loaded piston that determines how much pressure goes fore and aft. Each valve is calibrated for a specific application by the vehicle manufacturer, so it's important to make sure you get the correct replacement valve for a customer's vehicle if the original valve is defective. The calibration of the proportioning valve is fixed and cannot be adjusted.

Some vehicles have "load sensing" proportioning valves that change rear brake metering to compensate for changes in vehicle loading and weight shifts that occur during braking. This type of proportioning valve has an adjustable linkage that connects to the rear suspension or axle. As the vehicle is loaded, ride height decreases and pressure to the rear brakes is increased. This type of proportioning valve can be found on many minivans, pickups and even some passenger cars (Ford Tempo or Mercury Topaz, and Ford Taurus or Mercury Sable to name a few).

Load sensing proportioning valves are adjustable, and must be adjusted correctly if they are to properly balance the rear brakes to the vehicle's load. The valve linkage is adjusted with the suspension at its normal height (wheels on the ground) and the vehicle unloaded. The adjustment bracket or linkage is then adjusted according to the vehicle manufacturer's instructions, which typically involves adjusting the linkage to a certain position or height.

Load sensing proportioning valves are also calibrated to work with stock springs. Any suspension modifications that increase the load carrying capability (installing helper springs, overload or air-assist shocks for example) may adversely affect the operation of this type of proportioning valve. Modifications that make the suspension stiffer reduce the amount of deflection in the suspension when the vehicle is loaded, which prevents the proportioning valve from increasing rear brake effort as much as it normally would.

A defective proportioning valve or one that is not adjusted properly can also upset brake balance. So if the rear brakes on a vehicle seem to be overly aggressive (too much pressure to the rear brakes), or the vehicle seems to take too long to stop (not enough pressure to the rear brakes), the problem may be a bad proportioning valve. Proportioning valves can be tested by installing a pair of hydraulic gauges (one on each side of the valve) to see if the valve reduces pressure as it should.

On some newer vehicles, the mechanical proportioning valve has been replaced by "electronic" brake proportioning which is controlled through the ABS system. By sensing wheel speeds, the ABS system reduces pressure to the rear brakes as needed to keep them in proper balance with the front brakes.

METERING VALVE
Another device called a "metering" or "hold-off" valve is used in many vehicles that have front disc brakes and rear drum brakes to compensate for physical differences between the two different types of brakes. The metering valve holds off or delays the application of hydraulic pressure to the front disc brakes until the rear drums start to work. This is necessary because disc brakes don't have return springs, and consequently start to apply as soon as hydraulic pressure reaches the calipers. But the shoes inside the rear drum brakes have to first overcome the resistance of the return springs before they engage the drum. The metering valve (or valves) is usually located in the hydraulic lines to the front brakes.

PRESSURE DIFFERENTIAL VALVE AND BRAKE WARNING LIGHT
Located between the front and rear (or diagonally split) brake lines in most brake systems is a "pressure differential valve." This valve contains a switch that illuminates the brake warning light to warn the driver in case either side of the brake system loses pressure.

The valve has a piston that remains in a neutral position as long as pressure on both sides of the hydraulic system are equal. A loss of fluid on either side causes the piston to slide to one side when the brakes are applied, which completes the electrical circuit to illuminate the brake warning light. The valve is self-centering on most applications, but must be reset on others if a problem has occurred.

COMBINATION VALVES
On many vehicles, a "combination valve" is used, which contains a pressure differential valve, proportioning and/or metering valve. The combination valve performs the same function as these other valves in a single unit.

BRAKE LINES AND HOSES
The arteries of the brake system are the steel lines and flexible rubber hoses that route hydraulic pressure to each brake when the driver steps on the brake pedal. The lines and hoses must withstand pressures that can range from a few hundred pounds per square inch up to almost 2000 psi! If a line or hose can't take the pressure and blows, all braking ability in the affected brake circuit will be lost. A slow leak in a brake line or hose is almost as bad as a sudden failure because over time, enough fluid may be lost to allow air to enter the hydraulic system. Air in the fluid is bad because air is compressible. This increases the amount of pedal travel that's necessary to apply the brakes, and may increase it to the point where the pedal hits the floor before the brakes apply.

The first indication of a leak in a brake line or hose may be a low fluid level in the master cylinder reservoir. Other clues may include wet spots on the driveway, dampness on the back of a drum brake, or a brake warning light that comes on. If a leak is suspected, the entire brake system should be inspected for a leak. The most likely leak points are the brake calipers, wheel cylinders and rubber brake hoses, though steel lines can also rust through and leak. Fluid can sometimes be pulled into the engine through a leak in the power brake vacuum booster. If there's any fluid inside brake booster vacuum hose, the brake booster needs to be replaced.

Rubber brake hoses also need to be inspected for age cracks, bulges, swelling or other damage that would indicate a need for replacement. Rubber hoses have an expansion resistant inner lining that should not give under pressure. If the inner liner leaks, fluid will force its way under the outer liner causing a bubble or blister to appear when the brakes are applied.

Although it doesn't happen very often, sometimes, internal damage or deterioration in a rubber hose allows a small flap of material to lift up and plug the line. This prevents brake pressure from reaching the wheel causing a brake pull when the brakes are applied.

The same thing can also happen to steel brake lines. Debris in the brake fluid or a crushed or kinked line can block the passage of hydraulic pressure to the brakes. In some cases, pressure will get through but when the brakes are released, the blockage prevents pressure from releasing back to the master cylinder, causing the brake to drag.

REPLACING BRAKE HOSES AND LINES
Replacement hoses must be the same lengths as the originals, with the same type of end fittings. A hose that is too long may rub against the suspension or other parts, while a hose that's too short may be stretched and damaged when turning or by excessive suspension travel.

Most hoses have a male fitting on one end and a female fitting on the other. If a hose fitting has a copper gasket, it should also be replaced when changing the hose.

If a steel brake line is being replaced, it must be "approved" double-walled, welded steel tubing because of the high pressure the line must hold. No other type of tubing should ever be used for a brake line (for example, copper or aluminum tubing.) It's also important to replace the original brake line with one of the same size. Installing a larger or smaller diameter line may alter the brake balance in the affected circuit and upset the braking characteristics of the vehicle. Tubing diameters are commonly 3/16 in. (4.8 mm), 1/4 in. (6.4 mm) and 5/16 in. (7.9 mm).

The ends of the steel brake lines are either double flared or use an ISO (International Standards Organization) flare. The ISO flare is designed to deform when the fitting is tightened, which creates a more uniform seal and lessens the danger of overtightening. One type of flare is not interchangeable with another. The replacement line must have the same kind of flare as the original to seal properly.