turned out it was engine bay heat that killed the check valve (krank vent). i put a brand new one in over the weekend and within 30 minutes of driving, it too had stuck open. Fortunately i was able to get it working perfectly once it cooled down.
i ended up re-routing my catch can tubing to avoid the high heat in the valve, and also added a heat reflective sleeve over it. so far it's all good.