Fork seals should be replaced at the first sign of oil appearing on the stanchion tube legs. At first, there will be a black ring of oily dirt at the top of the fork's travel. If allowed to go on unchecked, the leak will worsen until there is enough oil accumulated on top of the dust seal to run down the back of the fork leg, straight onto the brake calipers. Eventually the fork will run out of oil, seizing the bushing surfaces that allow the fork halves to slide, thereby locking up the front end.