It sounds as though it's getting heat affected and perishing.
It could fail suddenly with fatal consequences, or it could bulge at the weakest point giving you some warning of imminent failure.
Impossible to predict either way.
In answer to your question about the material specs of the tubing.
The genuine item is manufactured to a particular standard with materials that are specifically designed for the intended purpose and in a manner that is compliant with the relevant standard.
You have 2 viable options.
1, Is to replace it with the genuine item sourced from a Toyota dealer.
2, Is to have one fabricated from braided brake tubing that has a Teflon liner and swaged fittings on each end which replicates the original item if it is no longer available.
The swaged fittings will most likely be a requirement but dependant upon the design rules applicable to the market into which the vehicle was sold.
The braided tubing is also designed for the intended purpose. Goodrich braided brake line tubing comes to mind.
It's type is typically used on race cars and universally on modern high performance motorcycles.
Good luck.