Which system is commonly noted as NOT using a filter?

The brake master cylinder is typically the correct choice when identifying a system that does not use a filter. The primary function of the brake master cylinder is to convert the force exerted on the brake pedal into hydraulic pressure, which then engages the brake components to slow or stop the vehicle. While it contains brake fluid, there is no separate or dedicated filter involved in this process.

In contrast, the engine cooling system, fuel system, and oil lubrication system all incorporate filters as essential components. The engine cooling system utilizes a radiator and coolant that may require periodic filtering to remove debris. The fuel system has fuel filters that ensure contaminants do not reach the engine. The oil lubrication system employs oil filters to prevent particles from circulating within the engine, ensuring proper lubrication and longevity of engine components. Thus, the brake master cylinder stands out as the system that operates without a filtration mechanism.