per this website, here is a better explenation
What Is the Purpose of the Breather Tube on a Rack & Pinion? | eHow
The Bellows and the Breather Tube
Both bellows at the ends of the rack assembly are sealed by clamps. If there were not some form of breather mechanism installed in the system, the air held inside the bellows near the inside of the turn would be compressed, blowing up the bellows like a balloon. At the same time, there would be too little air inside the opposite bellows, causing it to collapse inward and become trapped and torn by the moving mechanical parts.
It is not possible to use external air to prevent these problems, because it would likely be contaminated with dust and grit that would consequently be introduced to the machinery inside the bellows. The solution is the breather tube; it runs between the two bellows, allowing the excess air from the inside bellows to be vented to the outside bellows, where it stops the collapse. When the car is going straight forward, the breather tube allows the two bellows to equalize.
What Is the Purpose of the Breather Tube on a Rack & Pinion? | eHow
The Bellows and the Breather Tube
Both bellows at the ends of the rack assembly are sealed by clamps. If there were not some form of breather mechanism installed in the system, the air held inside the bellows near the inside of the turn would be compressed, blowing up the bellows like a balloon. At the same time, there would be too little air inside the opposite bellows, causing it to collapse inward and become trapped and torn by the moving mechanical parts.
It is not possible to use external air to prevent these problems, because it would likely be contaminated with dust and grit that would consequently be introduced to the machinery inside the bellows. The solution is the breather tube; it runs between the two bellows, allowing the excess air from the inside bellows to be vented to the outside bellows, where it stops the collapse. When the car is going straight forward, the breather tube allows the two bellows to equalize.
