How Brakes Work, Thanks to an Old Jeep CJ

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Jalopnik guts a Jeep CJ to show the science behind what makes our vehicles stop.

Some teach by reading from a book. Others teach by telling stories. Then there are those who teach by being hands-on and getting some dirt under their nails. Jalopniks David Tracy is that kind of teacher, and he uses an old Jeep to educate his viewers.

Dissected Jeep CJ

In the above video, the former FCA man who led the design of the powertrain cooling system for the JL Wrangler is on a mission to find a junkyard vehicle with a brake system that’s easy to extract.

He plans to use it to show how brakes work from pedal push to full stop. Tracy comes across a CJ and proceeds to remove its brake booster, master cylinder, brake pedal assembly, and rotor/caliper setup. He takes the dirty, forgotten hardware back to his shop, rust, and gunk staining his hands.


ALSO SEE: What the Forum Has to Say About this Breakdown


Then he breaks things down. Here’s the gist of his explanation:

1.) When you push down on your brake pedal, the long lever arm it’s connected to gives you a mechanical advantage that’s useful when trying to push a rod through your brake booster.

2.) So is the negative pressure created by your engine’s piston’s moving down to suck in air. The pressure pulls the rod even further through the booster and into the brake master cylinder.

3.) That has a piston that pushes brake fluid through the metal brake lines that eventually connect to rubber lines, which feed into the brake pistons. Given that pressure is force over area and the brake pistons have a larger area than the master cylinder’s piston, they deliver another mechanical advantage. They use that to make the calipers clamp the brake pads down on the discs and bring the vehicle to a halt.

Tracy teaches an important and interesting lesson. We would say someone should teach him one about hand cleaners that can take rust and grease off, but we have a feeling Tracy already knows it well.

Derek Shiekhi contributes to a variety of Internet Brands’ Auto sites, including J-K Forum, Jaguar Forums, and 5 Series. He's also a member of the Texas Auto Writers Association.

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