How a Brake Booster and Master Cylinder Work

Here's how a brake booster and master cylinder work to stop your vehicle with the press of your brake pedal.

Stopping a heavy, 2000+ lb car is no easy task. It takes a lot of force to press up against the discs and drums on each wheel to stop a car quickly.

The hydraulics of brakes is quite simple. A primary piston (known as the master cylinder) pushes fluid into hydraulic lines that get fed out to the wheels. The piston inside the caliper (or wheel cylinder inside the drum) will expand with the fluid, causing it to glide up against the disc (or drum), slowing the wheel.

The brake booster was developed to sit in between the master cylinder and driver's pedal, to make it easier for it to press the pedal. While the master cylinder's diameter is already smaller than that of the caliper pistons, the force required to compress it is still great.

The brake booster works on the principle of vacuum differentials to aid in pushing the master cylinder. On one side, vacuum is sucked from the engine's intake. At idle, a valve in the diaphragm allows vacuum to be passed through the diaphragm, so that vacuum balances both sides.

When you depress the brakes, that valve moves, sealing off the vacuum side, while allowing filtered atmospheric air to enter the booster from the brake pedal side. This creates a pressure differential between the diaphragm, which helps to force the piston in the master cylinder to compress.

A giant return spring brings the diaphragm back to its rest position when the brake pedal is released.

The master cylinder consists of two small pistons in series. Each piston routes to two diagonally opposite wheels, for redundancy in case one springs a leak or the seal is compromised. Reserve brake fluid is contained in a reservoir above the master cylinder and is sucked into the piston assembly when the brake pedal is pushed.

Periodic brake flushes are required because brake fluid is hydroscopic and will absorb moisture and loose its effectiveness over time. Furthermore brake fluid will wear down with heat, and may become contaminated.

Purchase tools used in this video to dissect your own car parts:
Angle Grinder (aka snap ring pliers):
3/8" Ratchet set:
Screwdriver set:

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[speedkar]

Purchase tools used in this video to dissect your own car parts:
Angle Grinder (aka snap ring pliers):
3/8" Ratchet set:
Screwdriver set:

[chrise]

Dude, your a freaking natural. The key words you use, the pace, the brief definitions, the smooth demos of moving parts and disassembly. This is what content should be like!

[JosephAlukka]

The best explanation on any video I have seen. No shitty music, no intro bullshit, straight to the point and clear explanation!

[ZPFunk]

Ive watched hundreds of hours of turorials on a wide range of subjects. This was so clear, so concise, without any fluff. I've never come away feeling as informed as I am after this one. Thank you.

[HectorDomino.]

This guy knows how to explain, and doesn't care to destroy a part to teach. Thanks for your effort man.

[ronchang]

Wow, the editing is perfect! No need for skipping forward: constant flow of useful information. The humor thrown is on point. Thanks!

[jackmehoff]

Excellent tutorial Dude. No BS. No fluff. No stuttering and searching for the right words. No talking just to hear yourself talk. Straight to the point. Like it!

[petripat]

That snap ring plier multitool is great

[FantasyLegacyGraphicArts]

You have a gift for explaining mechanics in a way that is very clear, concise and easy to understand.

[jimmaynard]

I've been working on vehicles for 35 years and that is by far the most detailed explanation of how this system operates. Excellent job! Well done!

[JasonTAho]

Toothbrush for a pointer and old underwear for a rag, NICE!!

[COTU]

Another awesome video. For me, I appreciate how you just cut right to the chase. You take the time to edit your videos so that they are succinct and to the point. So many other YouTube presenters seem like they spend so much time talking and wasting time and not getting to the point of why the video was made to explain in the first place. Keep up the great work!

[davedumas]

lol the best snap ring pliers ive ever seen (made me chuckle)

[rodjohnson]

This is one of the very best instructional videos I've seen anywhere. Perfect presentation, clear, well-lit video, tons of info packed into only 7 minutes. Well done! You should get a job as an instructor/teacher!

[richmac]

This is one of the best videos I've ever seen explaining how the brake booster and master cylinder work. I always wondered why the brake pedal would return when there was vacuum on one side only - reason it does is because there is a valve that I didn't know about equalizing the vacuum on both sides. Thanks for explaining everything and for showing (rather than just talking) about the components.

[geoffhurley]

I LOL'd at your "snap ring pliers". Been replacing brake boosters for 30 years and never seen the insides. Nice!

[midhun-]

This is how a tutorial video supposed to be!!!

[aleksandarmiloeski]

Just wanted to say thank you SO much! Goes without saying, but this is by far the best video on the Internet explaining the brake system and its components.

[PH-mdxp]

Great explanation. What’s impressive in addition to your skilled explanation, is the design engineers that design these systems that we all take for granted in so many areas of modern life.

[anonymous]

That redundancy in the master cylinder saved my butt when merging from one highway to the other at 60mph. Brake line to the back wheels exploded at the same time.
I didn't know that until now.
Great video! Love the toothbrush!