Here's why:
* Air in the system: Replacing a brake line introduces the possibility of air entering the system. While you might be meticulous, a small amount of air can easily get trapped, especially in the furthest caliper from the master cylinder. This air can compromise braking performance, leading to a spongy brake pedal or even brake failure.
* Bleeding sequence: Brake bleeding follows a specific sequence (typically starting with the furthest wheel from the master cylinder and working your way closer) to efficiently purge air. Bleeding only one wheel might not remove all the air.
* Prevention is better than cure: The extra time spent bleeding all four wheels is a small price to pay compared to the potential consequences of a partially air-locked system.
In short, while you *might* get away with bleeding only the affected brake line, the risk is simply not worth it. It's far safer and more effective to bleed all four wheels to ensure a completely functional and safe braking system.
