Final answer:
Hydraulic brake systems require double wall thickness steel tubing to handle high pressure and prevent corrosion.
Step-by-step explanation:
The type of tubing that must be used in hydraulic brake systems is double wall thickness steel tubing. This kind of tubing is essential to ensure safety and reliability because it needs to withstand high pressure without bursting and resist corrosion that can lead to failure. The use of double-walled tubing in brake systems is an industry standard for these reasons.