Ans: It is not necessary to run the engine when bleeding the brakes. During brake bleeding, the air is removed from the brake lines to ensure the brakes are working efficiently. The engine should be off when doing this.  An engine running can make it difficult for the brake system to get rid of air since it needs a vacuum boost from the engine. If you press the brake pedal repeatedly until it feels firm, you will be able to remove air from the brake lines.