What should be checked to confirm that the brake pedal feels firm when the engine is off?

To confirm that the brake pedal feels firm when the engine is off, it is essential to check the power assist operation. When the engine is turned off, the power brake booster is not providing assistance, so the brake pedal should still feel firm under foot pressure. If the pedal feels soft or sinks to the floor, it indicates a potential issue with the brake system, such as a leak or a malfunction in the hydraulic system.

The other factors listed, such as fluids in the master cylinder, engine oil level, and brake pad thickness, are important for the overall functionality and safety of the vehicle but do not directly relate to the firmness of the brake pedal when the engine is not running. The master cylinder ensures that there is adequate brake fluid to operate the brakes, and while a low fluid level might affect brake performance, it does not directly assess the pedal's firmness without the engine assist. Engine oil level is unrelated to braking performance, and brake pad thickness pertains to how much friction material remains on the pads, which affects stopping power rather than the immediate pedal feel.