Compression release brakes operate by releasing ____________.

The operation of compression release brakes, often referred to as "jake brakes," primarily involves the release of combustion gas. When the engine is in a compression cycle, the intake valves remain closed, and air is compressed within the cylinders. During the braking process, these brakes open the exhaust valves at the top of the compression stroke, allowing the compressed combustion gases to escape.

As the combustion gases are released, it creates a significant reduction in engine power and slows down the vehicle. This action enhances the engine’s braking capability without applying the traditional braking system, leading to longer brake life and better control, especially on downhill slopes.

In this context, while compressed air and exhaust gas may seem similar, it's specifically the combustion gas that is involved in creating the braking effect for compression release brakes. The other options pertain to different systems in the engine that do not directly contribute to the braking mechanism involving compression release.