Okay, I'm ready! Someone will label me as a Sexist after my explanation.

Usually when we look for an assistant for bleeding brakes, it is our wives, girlfriends or kids. They probably have the strength, but don't understand the word EFFORT. They don't press very hard on the pedal.

By starting the engine, you now have the brake booster applying the needed EFFORT.

Some vehicles are not very difficult to overcome the resistance of the brake booster with the engine off. Others seem to have too much resistance from the booster, and that 95 pound lady just can't or won't overcome it.

Okay, so I'm a Sexist, arrest me!