put a couple of drops of dish soap in a juice glass. Fill it with water. Use a pastry brush, or a paint brush, and "paint" the soapy water onto all the connections... both ends of the check valve, the cap of the reactor, the cap of the gas separator if you have one, etc. If there is a leak, the soap in the water will cause it to "blow" big bubbles. If there is no leak, there will be no bubbles.
With the check valve, I would replace it anyway, just to be sure it isn't the issue. Easy thing to try for less than a buck.