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A compression release engine brake, frequently called a Jake brake or Jacobs brake, is an engine braking mechanism installed on some diesel engines. When activated, it opens exhaust valves in the cylinders, releasing the compressed air trapped in the cylinders, and slowing the vehicle.

Although Jake brake properly refers to the Jacobs brand of engine brakes, the term has become a genericized trademark and is often used to refer to engine brakes or compression release engine brakes in general, especially on large vehicles or heavy equipment.

They are often not allowed in to be used in towns as they create loud, clattering noises.
I saw a semi get pulled over because he pulled his Jake Brake in city limits trying to slow the rig down in a slower speed zone.
by Pele Lucciano October 16, 2010
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Short for the Jacobs Engine Brake, a secondary braking system used on large deisel trucks like fire engines. Used with automatic transmissions, usually on trucks that carry a heavy load. The system uses the compression of the engine as resistance to slow the drivetrain. Beneficial because it reduces wear and overheating problems with brakes. Causes the engine to make a great deep sound that can be heard all around.
I would have burned the brakes right off that rig if it weren't for the Jake on there.
We were going down the hill in this neighborhood and when the Jake Brake kicked, the sound set everyone's car alarm off.
by Firemaneric April 17, 2006
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{jayk-brayk} noun, 1. A person that interferes and/or prevents a hit or pick-up 2. A cock-blocker (us. female)
No I didn't get her number - the frickin' jake-brake pulled her out to the dance floor just when I started talking to her.

Damn, the jake-brake told her that I was married.
by Stupid Is As Stupid Does September 15, 2009
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