Fuel lines, water lines, air and fluid brake lines after a while can succumb to the elements.
Sun, dirt, road salt, water, cleaning chemicals etc, they all have an effect on these components.
Rubber will break down after a while and may look ok on the outside.
When manufactured, hoses are made in layers, especially if there is a braid inside them.
After a while the lining layer can break down seperating the layers and leaking out.
Ever seen BUBBLING on the outside of a brake hose or fuel line?? Thats it!
The opposite can also happen and instead of causing bubbling the vaccum in the line can cause the internal wall to collapse and block/restrict the flow.
Always check these hoses on a regular basis, they can go at any time, no warning especially now summers here and the rain is getting warmer....!!!!!