Gas down the side of your Pig is no fun!
That check valve you found is supposed to let positive pressure vent out of the vapor separator (black thing w/ 4 vertical tubes), but it is not supposed to let air back into the tank if the tank develops a negative pressure (think a warm tank of fuel cooling off at night). Your gas cap is supposed to enable the tank to draw air in, but not let pressure out.
You could start by checking the lines running in and out of the vapor separator. There should be 4 lines venting the tank running to the separator. If you can slide those loose from the separator, you can remove the gas cap and then run some low-pressure compressed air into each line and make sure the tank can vent into the separator. You should be able to hear the air escaping up through the fuel inlet.
If all is good there, you can "check" the check valve (ha ha). Then you can try pushing some low pressure air up through the line to the charcoal canister. Listen for the air up at the charcoal can.
Obviously, any obstructions need to be dealt with.
My check valve was cracked, and let fuel vapors into the passenger compartment
Let us know what you find!
