Normally the valves will start to grow hard and then leak air, when they are exposed to the clima in ex. Scandinavia where we use A LOT of salt on the road.
It simply just kills the rubber.
Last week, when I was working on a Mercedes Sprinter 312 I found a tire where a 30x10cm piece of outer rubber was missing on the inside of the tire - It was close to an eksplosion.
Check your valves and tires often, as soon as the first crack apears - Change it!
I change my valves everytime I change tires - Wich will be around 12 months on a tire and a valve (Everyday slave-scooter!).
/Swobber says: Rubber will start to harden and then crack.