why can't we make a hydraulic brake pipe that does not corrode and fail the M O T test?

Answer 1

We can and do. Brake pipes are made of stainless steel. They rarely corrode from the outside. Most brake pipe corrosion occurs from the inside! If moisture gets into the brake system, the extreme pressures in the pipes greatly accellerates the corrosive action on the inside of the brake pipes.

Brake fluid is hygroscopic. That means that it very readily absorbs moisture from the ambient air. Even opening the reservoir to check the fluid level periodically will cause moisture to get into the system. For that reason, most vehicles use a transparent or translucent reservoir so that you can check the level without removing the cap.

It's also possible that either you or your mechanic has been adding fluid in a misguided attempt to compensate for the dropping level in the reservoir. Modern disc brake systems will show a reduced level of fluid in the reservoir as the pistons in the caliper are pressed out to compensate for pad wear. This is NORMAL and you should never add fluid as long as the level is above the MIN mark on the reservoir!

A very common entry point for moisture is using contaminated brake fluid. When a container of brake fluid is opened, you should use it immediately and discard any remaining fluid after 48 hours. Remember, brake fluid has an extreme affinity for water. Once the container is opened there is no way to stop the moisure absorption by the fluid. You MUST discard the remaining fluid after no more than 48 hours!

Some manufacturers (Mercedes for one) require that brake systems be completely flushed every 2 years to prevent the accumulation of moisture in the system. This is a wise practice for ANY vehicle.

You shoud never need to add fluid to the brake reservoir. As long as the level is above the MIN mark on the side of the reservoir, LEAVE IT ALONE.

On the rare occasion that you need to add fluid, only use fluid from a freshly opened container and immediately discard the unused fluid.

And seriously consider having your brake system flushed with new fluid every 2 years.