Ok, whats the check valve made of?...
Some silicone formulas are resistant to petroleum / gasoline... if that pump was intended for medical use, it probably has a food grade silicone membrane... its a shame there isn't a small inexpensive fuel pump about the same size... then you could just modulate the fuel pressure, and leave the fuel tank vented to the atmosphere... it would only require a small change in fuel pressure to mimic muffler tank pressure.