Small Engines (Lawn Mowers, etc.): leaking upper crankshaft oil seal, ring gaskets, vacuum pump

Question
I HAVE A B/S 12.5 HP VANGARD V TWIN.MODEL 290777,TYPE 0410-01,CODE 91083011. (5526)IT IS LEAKING QUITE A BIT OF OIL AROUND THE SEAL
I CAN REPLACE THE SEAL OK.
MY CONCERN IS WHAT CAUSED THE SEAL TO LEAK.
MY QUESTION IS  HOW CLEAN /CHECK THE CRANKCASE VENTILATOR?
THANKS
RAY

Answer
Dis-assemble and clean the breather with carb cleaner.  Check the dipstick tube gasket and dipstick o-ring gaskets...make sure they are sealing.

Do you have a vacuum pump or manometer?  What weight oil are you using?  Does the oil smell like gas?

Eric