Cars · Lesson 8 of 15Intermediate9 min read

Brakes

Brakes turn the energy of a moving car into heat. Your foot supplies only a small push; a lever and a hydraulic system multiply it into thousands of pounds of squeeze at the wheels.

Pascal’s principle

Push on a liquid sealed in a container and the pressure rises the same amount everywhere in it. That’s Pascal’s principle. Pressure is force divided by area, so a small piston and a large piston connected by a line feel the same pressure, and the large one pushes with more force: force out ÷ area out = force in ÷ area in.

There’s no free lunch. The large piston moves a shorter distance, so the work out is no more than the work in. OpenStax’s physics textbook works an example: a 100-newton push on the pedal becomes 500 newtons at the pedal cylinder through a lever. With a pedal cylinder 0.500 cm across and wheel cylinders 2.50 cm across, each wheel cylinder pushes with about 12,500 newtons, 25 times more.

F_out = F_in × (A_out ÷ A_in) and since A = π d² ÷ 4, the force grows with the square of the diameter ratio

Change the push, the lever and the piston sizes. The starting values reproduce the OpenStax example. Real cars add a power booster, so these are physics numbers, not specifications for any car.

From pedal to pads

The brake pedal is a lever that pushes a piston in the master cylinder. Modern cars use a dual (tandem) master cylinder with two pistons and two circuits, split front-and-rear or corner-to-corner, so a leak in one circuit leaves the other still working. A warning switch lights the brake light on the dashboard if the pressure in the two circuits gets out of balance.

From the master cylinder, double-walled steel lines carry fluid toward each wheel, with flexible rubber hoses where parts move, such as to the front calipers. A power booster adds force to your push; most use engine vacuum, and a steady hiss when you press the pedal can mean a vacuum leak.

Disc and drum brakes

A disc brake has a rotor, a metal disc that turns with the wheel, and a caliper that straddles it. Fluid pressure pushes the caliper’s piston, and the pads squeeze both sides of the rotor. A floating caliper slides sideways on its mounts; a fixed caliper has pistons on both sides. Many pads have a small metal wear indicator that squeals or screeches against the rotor when the pads get thin. Pads and rotors are replaced in pairs, both sides of an axle.

A drum brake has two curved shoes inside a drum that turns with the wheel. A wheel cylinder pushes the shoes outward against the drum. The drum’s rotation tends to drag the leading shoe in tighter, a self-energizing effect that adds to the braking force.

Both turn the car’s motion into heat by friction. Source: U.S. Navy Construction Mechanic Basic, Chapter 13.

Brake fluid

Brake fluid has to stay liquid and resist boiling even when the brakes get very hot, because boiling would make gas bubbles, and gas squeezes where liquid doesn’t. Most fluids (DOT 3, DOT 4 and DOT 5.1) are glycol-based and absorb moisture from the air over time, which lowers their boiling point. That’s why the federal standard requires every container to say: “Keep container clean and tightly closed to prevent absorption of moisture.” DOT 5 is silicone-based and dyed purple. Use only the type printed on your reservoir cap or in your owner’s manual.

Minimum boiling points required by FMVSS No. 116
Grade	Fresh (dry)	After absorbing water (wet)
DOT 3	401°F (205°C)	284°F (140°C)
DOT 4	446°F (230°C)	311°F (155°C)
DOT 5	500°F (260°C)	356°F (180°C)

ABS, parking brakes and regenerative braking

A locked, skidding wheel can’t steer. Antilock brakes (ABS) watch each wheel with a speed sensor. When a wheel starts to lock, the ABS computer opens a valve to release that wheel’s pressure for an instant, then reapplies it, keeping the tire just short of skidding, where grip is greatest and you can still steer.

The parking brake is a separate, mechanical way to hold the car, on many cars a cable to the rear brakes. Hybrids and electric cars also brake regeneratively: the motor runs as a generator, turning motion back into electricity for the battery. That captures energy normally lost as heat and significantly reduces brake wear, but the friction brakes are still there for hard stops.

Parts and what they do
Part	What it does	Signs of trouble
Master cylinder	Turns pedal force into hydraulic pressure, in two circuits.	A pedal that slowly sinks to the floor when you hold it.
Power booster	Adds force to your push.	A hard pedal; a hiss when you press it.
Lines and hoses	Carry fluid to each wheel.	Wet, dark spots; cracked or chafed hoses.
Calipers	Squeeze the pads against the rotor.	Pulling to one side when braking.
Pads	Friction material that presses on the rotor.	Squeal from the wear indicator; grinding when worn through.
Rotors	Discs that turn with the wheel and soak up heat.	Scoring, heat cracks, warping.
Drums and shoes	The other way to make friction at the wheel.	Scoring and heat cracks; linings worn thin.
Brake fluid	Carries pressure; must resist boiling and corrosion.	Low reservoir level (pads wearing or a leak), a soft pedal.
ABS unit and sensors	Prevent wheel lockup.	The ABS warning light.

You canWhat you can safely do

Check the brake fluid level at the reservoir (Check Coolant, Brake Fluid, and Washer Fluid).
Listen for squealing from the wear indicators and look at the pads through the wheel if you can see them.
Never blow brake dust away with compressed air, and don’t clean brakes with a dry rag or brush. Some brake parts may contain asbestos, and you can’t tell by looking. EPA suggests home mechanics use a wet-wipe method: mist the parts with water, then wipe them clean.

StopWhen to stop driving

Have the brakes checked before you drive further if the pedal feels spongy, sinks toward the floor when you hold it, or feels much harder than usual; if you hear or feel scraping or grinding; if the car pulls to one side when you brake; or if the red brake light stays on. A spongy pedal usually means air in the lines; a sinking pedal can mean a leak inside the master cylinder.

Check yourself

Tap an answer to check it, or open “Show the answer.”

What does Pascal’s principle say?
Show the answer
Pressure in a sealed liquid changes the same amount everywhere. A change in pressure on an enclosed fluid is passed on undiminished to every part of it.
A caliper piston has twice the diameter of the master cylinder piston. How much more force does it push with?
Show the answer
Four times as much. Area grows with the square of diameter: 2² = 4 times the area, so 4 times the force at the same pressure.
Why do most brake fluids need replacing over time?
Show the answer
They absorb moisture, which lowers the boiling point. Glycol fluids absorb water from the air, and the wet boiling points are much lower than the dry ones.
Why do cars have two brake circuits?
Show the answer
So a leak in one still leaves some braking. The dual master cylinder keeps one circuit working if the other fails.
What does ABS prevent?
Show the answer
Wheel lockup and skidding. By releasing and reapplying pressure, ABS keeps the wheels turning so you can still steer.

Go deeper

OpenStax College Physics 2e, 11.5 Pascal's Principle Pascal’s principle with the hydraulic brake example.
U.S. Navy, Construction Mechanic Basic (NAVEDTRA 14264A), Chapter 13: Brakes The Navy’s full chapter on hydraulic brakes.
Clemson University Vehicular Electronics Laboratory: Antilock Braking Systems How antilock brakes work, from Clemson University.
U.S. EPA: Current Best Practices for Preventing Asbestos Exposure Among Brake and Clutch Repair Workers EPA’s brake and clutch dust guidance, including for home mechanics.

Sources

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A free course from Teddy Rook Book. It explains how cars work in general and follows U.S. sources; your owner’s manual and your vehicle’s service information are the final word for your car, along with state and local rules.