After every 2 or 3 starts, I spray some WD40 into the valve to keep it lubricated. The propane freezing makes the valve stick so it stays open longer than it normally would which is why you get so much gas escaping. The WD40 stops this sticking so when I disconnect from the valve it shuts almost instantly - admittedly I still get a tiny bit of gas escaping but I don't think you'll ever get around that!