What is NOT a type of retarder?

Retarders are devices used to slow down a vehicle without using the traditional braking system. Each type of retarder operates differently.

Exhaust retarders work by redirecting the vehicle's exhaust gases, creating back pressure in the engine, which helps slow the vehicle down. Engine retarders, often called compression release engines or Jake brakes, utilize engine compression to create a braking effect. Hydraulic retarders use fluid dynamics to generate a resistance force that slows the vehicle.

The option that is not a type of retarder is the vacuum retarder. While vacuum systems may play a role in some vehicle braking operations, they are not categorized as a retarder in the same context as exhaust, engine, and hydraulic retarders. Therefore, choosing the vacuum system as the answer accurately distinguishes it from the established categories of retarders commonly utilized in vehicles.