Typical, medium-duty, hydraulic brake system with front discs (red circuit) and rear drums (green circuit). Boost, or assist (blue circuit), is provided by an engine-driven pump, although the power steering pump often performs this function. Parking brakes (orange circuit) are actuated by a dash valve.
Ever wonder why there can’t be just one kind of brake? It’s because air and hydraulic brakes each have operating characteristics that make one or the other ideal for certain applications.
In heavy-duty combination vehicles, air is the clear choice because of the large volume of liquid that would be needed to accuate all the wheel cylinders. Plus, dealing with gladhands and hoses filled with hydraulic fluid would be messy.
But for light and medium-duty straight-truck applications, hydraulic brakes offer advantages including:
- Brake feel – that is, as the pedal is pressed farther down, effort increases;
- High line pressures, which permit the use of lighter, more compact braking components;
- Less initial expense, due to smaller and fewer components;
- Cleanliness – hydraulic brakes are closed systems;
- Ease of locating leaks, since fluid is visible.
There are many more permutations of hydraulic brake systems than found in air systems, but all have basic similarities.
The hydraulic system
All hydraulic brake systems contain a fluid reservoir, a master cylinder, which produces hydraulic pressure, hydraulic lines and hoses to carry pressurized fluid to the brakes, and one or more wheel cylinder(s) on each wheel.
The wheel cylinders expand under fluid pressure, and force the brake shoes against the insides of the drums. If disc brakes are used, calipers, with integral cylinders, clamp down on the rotors when pressure is applied.
Because a vehicle must be able to stop much more quickly than it can accelerate, a tremendous amount of braking force is needed. Therefore, the retarding horsepower generated by the brakes must be several times that of the engine.
In order to develop the forces required to hold the brake linings against the drums or discs, and to achieve controlled deceleration, it is necessary to multiply the original force applied at the brake pedal.
When a hydraulic system is used, the only mechanical leverage is in the foot pedal linkage. However, varying the diameter of the wheel cylinders or caliper diameters, in relation to the master cylinder bore diameter, provides an additional increase in ratio.
In a hydraulic system, the pressure delivered by the various wheel cylinders is directly affected by the areas of their pistons. For example, if one wheel-cylinder piston has an area of 2 square inches, and another piston has an area of 1 square inch, and the system pressure
Brake shoes (left) are forced apart by the wheel cylinder, and rub against the inside of the drum to stop the vehicle. Disc brakes (right) use hydraulic pressure in the integral cylinder to force brake pads to clamp down on the rotor.
is 400 psi, the 2-square-inch piston will push against the brake shoes with a force of 800 pounds. The 1-square-inch piston will exert a force of 400 pounds. The ratio between the areas of the master cylinder and the wheel cylinders determine the multiplication of force at the wheel cylinder pistons.
Keep in mind that the larger a wheel cylinder’s diameter, the more fluid must be supplied by the master cylinder to fill it. This translates into a longer master-cylinder stroke.
If the master cylinder bore diameter is increased and the applying force remains the same, less pressure will be developed in the system, but a larger wheel-cylinder piston can be used to achieve the desired pressure at the wheel cylinder. Obviously, a replacement master cylinder, wheel cylinder or caliper must be of the same design and bore as the original unit.
Hydraulic brake systems are split systems, comprising two discreet braking circuits. One master-cylinder piston and reservoir is used to actuate the brakes on one axle, with a separate piston and reservoir actuating the brakes on the other axle(s). Although rare, some light-duty brake systems are split diagonally rather than axle by axle.
The reason for the split system is that if a leak develops in one hydraulic circuit, the other will stop the vehicle. Of course, the vehicle shouldn’t be driven any farther than necessary to have the brake system repaired.
When one of the hydraulic circuits fails, a pressure-differential switch senses unequal pressure between the two circuits. The switch contains a piston located by a centering spring and electrical contacts at each end. Fluid pressure from one hydraulic circuit is supplied to one end of the pressure-differential switch, and pressure from the other circuit is supplied to the other end. As pressure falls in one circuit, the other circuit’s normal pressure forces the piston to the inoperative side, closing the contacts and illuminating a dashboard warning light.
Power assist units, or boosters, reduce operator effort at the brake pedal. Vacuum boosters, popular on light-duty vehicles, make use of an engine vacuum on one side of a diaphragm, and atmospheric pressure on the other side. A valve allows the vacuum to act on the diaphragm in proportion to brake pedal travel. This assists the pedal effort, and allows increased pressure on the brake fluid, without an undue increase in pedal effort.
Other types of boosters use hydraulic pressure – either from the vehicle’s power steering pump or from a separate electric pump, or both – to assist pedal effort. As the brake pedal is depressed, a valve increases hydraulic pressure in a boost chamber to apply increased pressure to the master cylinder pistons.
Some systems use both vacuum and hydraulic assist. In other systems, air pressure from an onboard compressor is used to generate hydraulic system pressure.
Valves commonly found in hydraulic brake systems include:
- Proportioning, or pressure-balance valves. These restrict a percentage of hydraulic pressure to the rear brakes when system pressure reaches a preset high value. This improves front/rear brake balance during high-speed braking, when some of a vehicle’s rear weight is transferred forward, and helps prevent rear-wheel lockup. Some proportioning valves are height-sensing. That is, they adjust rear-brake pressure in response to vehicle load. As a vehicle’s load increases (decreasing height) more hydraulic pressure to the rear brakes is allowed;
- Metering valves. These hold off pressure to front disc brakes to allow rear drum brake shoes to overcome return-spring pressure and make contact with the rear drums. This prevents locking the front brakes on slippery surfaces under light braking applications. These valves do not come into play during hard braking.
The parking function varies greatly among hydraulic brake systems. Many light-duty vehicles with rear drum brakes use a passenger-car type lever-and-cable setup. A ratcheted lever or
Self-energizing feature of drum brakes. When brake shoes expand and contact the rotating drum, the leading brake shoe is pushed against the trailing shoe by force of the moving drum. This results in higher lining-to-drum pressure than would be produced by wheel cylinder(s) alone.
foot pedal pulls a cable, which, in turn, pulls a lever assembly at each rear wheel end. The lever forces the brake shoes apart, and they are mechanically held against the drums until the ratchet is released.
Other parking systems include spring chambers, like those used on air-brake systems. These are spring-engaged, but are disengaged by hydraulic pressure instead of air.
On many hydraulically braked light-duty trucks, antilock brakes are used on the rear wheels to preserve braking stability when these vehicles are lightly loaded. Front and rear-wheel antilock is usually an option, except for vehicles over 10,000 pounds GVWR, which are required to have steer and drive-axle antilock.
In current hydraulic antilock systems, a dump valve releases pressurized hydraulic fluid into an accumulator in the event of an impending wheel lockup.
An electronic control box receives speed signal(s) from sensors in the transmission and/or at the wheels. When the brakes are applied, the control box senses the decrease in rear wheel speed, and activates the dump valve(s) if the rate of deceleration exceeds a predetermined limit.
The control box energizes the dump valve with a series of rapid pulses to bleed-off wheel hydraulic pressure. Continuing in antilock mode, the dump valve is pulsed to keep the wheels rotating, while maintaining controlled deceleration.
At the end of such a stop, the valve de-energizes and any fluid in the accumulator is returned to the master cylinder. Normal brake operation resumes.
Foundation brakes in hydraulic systems can be either drum or disc. In many applications, discs are used on the front axle and drums on the rear.
Drum brakes are said to be self-energizing. That’s because when the brake shoes expand and contact a rotating drum, the leading, or forward, brake shoe is pushed against the trailing shoe by the force of the moving drum. This results in higher lining-to-drum pressure than would be produced by the wheel cylinder alone.
As brake linings wear, the shoes periodically must be moved closer to the drums to ensure proper contact during braking. While some older drum brake assemblies are manually adjusted, most are automatic. These use a star wheel or ratchet assembly, which senses when the wheel cylinder has traveled beyond its normal stroke, and expands the pivot point at the other end of the brake shoes.
In addition to being one of the friction elements, the brake drum or rotor also acts as a heat sink. It must rapidly absorb heat during braking, and hold it until it can be dissipated into the air. The heavier a drum or rotor is, the more heat it can hold.
This is important, since the hotter the brake linings get, the more susceptible they are to heat fade. Heat fade is induced by repeated hard stops and results in reduced lining-to-drum/rotor friction and increased vehicle stopping distance. As a rule, high-quality linings will display less heat fade than inferior ones. Also, disc brakes are far more resistant to heat fade than drum brakes.
Another type of fade that brakes are susceptible to is water fade. Drum brakes, with their large surface areas, apply fewer pounds per square inch of force between lining and drum during a stop than disc brakes. This, added to the drum’s water-retaining shape, promotes hydroplaning between shoe and drum under wet conditions. The result is greatly increased stopping distance.
Disc brakes, with their smaller friction surfaces and high clamping forces, do a good job of wiping water from rotors, and display little reduction in stopping capability when wet.