They are one-way check valves that also regulate the amount of vacuum that goes to a particular system (climate control, door locks, etc.).

Mikemover is right on with the analysis that your vacuum pump is failing and allowing oil into the vacuum system. You need to fix this as soon as is practicable.

Good Luck!
Tom

__________________
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
Formerly:
Shop Foreman;
Technical Advisor to Am. Honda;
Supervisor of Maintenance largest tree care co. in US for offices in Tex.