I never used after run oil, but would run FAI fuel (80% methanol and 20% oil; about 3% of the oil was castor) through the engine at the end of each flying session. Another thought for your problem: The check valve is just a simple ball valve. If the valve has been sitting for a long time it may not be working. You can soak it in alcohol to free up the ball.
Good luck - The YS short stroke is a great engine and they will last a long time!!
-Will B.