Rusted MSD

yes this is common for msd it's caused by ozone buildup . there is a way to deal with this problem , bore a small hole in the cap , fit a small brass orfice fitting and run a hose to the bottom of the breather , most msd caps have a vent . I have herd of some people running the hose to a small vacuum port on the carb but this would be imo a leak .