Review on 🛠️ Muhize 25 Ft. of 1/4 Brake Line Tubing Kit - Flexible Double Galvanized Steel Tube Roll 25 ft 1/4" (Includes 16 Fittings) for Enhanced Performance in Hydraulic Braking, Fuel, and Transmission Systems by David Robinson

I don't know what the material is or if rust is the right word for what happened. I bought this tube because I needed a copper tube for heat transfer (non-food grade) for my little hobby project. 24 hours (in air) after unpacking this hose I flushed the line with water and noticed a black residue coming out of the back. I looked closely and saw that black spots had formed on the pipe. I couldn't use this without risking damaging my flow pumps so I put it in a heap to throw away. A week later, the black pockmarks turned into whole black sections of the whistle. For the brake line I would recommend a hard pass for this product. Black deposits can easily clog nozzles or damage pumps. Blackened areas of the hose pose a leak/rupture hazard which is simply not necessary and definitely not what you need for a critical system like brakes.