Front fork oil leak

You notice a greasy film of oil on the shiny fork tube, a deposit running down toward the caliper, or a front wheel that dives and rebounds abnormally under braking. The telescopic fork is a hydraulic damper: inside each leg, oil flows through valves to slow the wheel's movement, and a fork seal (oil seal) keeps that oil from escaping along the stanchion. When the seal hardens with age, gets nicked on a spot of rust on the tube, or lets oil past because a grain of grit has lodged under its lip, oil weeps out: the fork loses damping, dives more under braking, bottoms out on bumps and — more seriously — throws oil onto the disc and pads, dangerously reducing braking. A leaking fork is an MOT failure and a real hazard: treat it quickly. Replacing the fork seals is within reach of a methodical DIYer on many models, but a pitted or bent tube (after a crash) is a job for a professional.