مانع تسرب أو حلقة O متهالكة

A rubber seal hardened, cracked or flattened with age and heat, so a joint that was sealed no longer is. It is deliberately a general cause, because the same failure appears across hydraulic, fuel, vacuum and coolant circuits and the mechanism does not change with the fluid.

Seals fail by losing compliance rather than by breaking. A seal works because it is squeezed and pushes back; heat cycling and time make the rubber hard, and it takes a permanent set in the shape of its groove. So the leak appears when cold and closes as things expand, which is the pattern to recognise and the reason an intermittent leak is so often a seal rather than a crack.

Two practical points. A seal is single-use — one that has taken a set will not reseal on reassembly, so any joint opened wants a new one, and reusing an O-ring is a common reason a repair leaks immediately. And the material must match the fluid: a seal correct for oil may swell or dissolve in brake fluid or a gaseous fuel, so a generic O-ring of the right dimensions is not necessarily the right part.