Fill and Vent Plug Leak

[cujo7098]

Hydraulic steering fluid leaks from my fill and vent plug on top of the steering column. This started after routine service to remove and replace the steering fluid. It did not ever leak prior to this service. The leaks occur when the steering column is exposed to higher temperatures and direct sunlight. This has been occurring for several months now, and only when exposed to heat from direct sunlight. The fluid in the fill port, has at this point, remained at an adequate level. This was the first time the fluid exchange was done, as the boat is only five years old. Any suggestions?