When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
What looks like a rubber line to you is actualy the outer covering of a woven inter core over some type of poly tubing made to handle very high pressures and live with brake fluids. The inner construction is made to stay pretty firm or you would feel a little softness in the pedal as long as there where no other brake system issues.
Yes they sometimes break through the inter core and baloon the outer covering if there is no break in the outer covering.
Leaks often develope at the metal fittings on the ends from age and flexing.
An upgrade to these line is stainless steel lines that have improved inner core and the strength of the SS outer covering.
SS lines are most often used on race cars.
The braided stainless lines are spendy but at least for the fronts they are worth it. I am not entirely convinced that rears are worth the money as the majority of the braking power is in the front but I know that the pedal feel on my Mustang increased tremendously with the addition of the stainless lines.