Small Engines (Lawn Mowers, etc.): Gas is leaking by, pet cock, defective valve

Question
I've got a John Deere stx 38 with a 12.5 engine. Problem is every time I shut the engine off, gas continues to leak by and eventually trickles down and settles into the muffler.  I'm getting lots of gas fumes in the garage area and after starting a lot of gas blows out the muffler.  I've replaced the float in the carb but that didn't do anything.  There is no apparent sign of gas getting into the oil and the tractor runs great at all times.  This has been going on for 2 seasons now.  I've been turning the gas valve off as I park it and let it run itself dry. Now just yesterday that seems to be not working anymore as the valve seems to be letting the gas by.  Any ideas on what this could be? Is this going to lead to more serious trouble?  

Answer
MARK, IF IT WERE ME I WOULD RELACE THE VALVE WITH A BETTER QUALITY VALVE SAY ONE FROM A MOTORCYCLE / MARINE SHOP OR SUCH.  I SIMPLY THINK IT'S A DEFECTIVE VALVE AND IT WILL OVER TIME CAUSE YOU TROUBLE ALLOWING THE GAS TO "SOUR" IN THE CARB.  THOSE LITLE PET COCK VALVES ARE PRETTY MUCH STANDARD SIZE AND YOU SHOULD HAVE NO TROUBLE FINDING A REPLACEMENT.

GOOD MOWING,
GEORGE