The black seat goes in the bottom of the freshly cleaned tool handle bore first. There is a black raised circle or 360 ring on the black seat that must face up toward you. Some seats have raised circles on both sides. Use the tallest side of the seat. (Blame Snap On for this issue as they changed the valve seat 4 years ago.) The circle ring is where the face of the black tip valve seals to prevent the tool from passing air or operating when your trigger is released. The seat is carefully installed using soft material like fuel line to push it into the tool bore or a thin wall 12 point metric socket with a 4" or 6" extension plugged into the socket.  Use oil or clean vaseline or silicone grease to lube the seat and help it settle into place with the help of your fuel line to push it home.  I next slide the trigger stem assembly using a flashlight to see into the bore to center the trigger stem pin locating hole (for the tip valve stem) straight down in the middle of the hole in the white tip valve seat. 

Make sure to locate the black tip valve stem into the trigger stem hole in the trigger stem through the black seat hole while holding it with your long tweezers. Test your work by placing the narrow end of the tip valve spring onto the tip valve nub. Apply holding spring pressure on the tip valve to feel it lift when you push in on the trigger stem. Slowly and gently push the trigger to feel if you correctly located the tip valve stem into the trigger valve stem hole below. If you immediately feel tip valve movement with immediate trigger movement then you are correctly located. Now leave the tip valve spring with the narrow end on the black tip valve nub. Change the old o'ring on the steel bushing that screws into the tool handle. There are 4 o'rings and 2 seats for 2 repairs. Screw the bushing into the tool applying pressure on the spring gently and slowly until you seat the bushing and tighten it. Avoid pulling the tool handle threads due to over-tightening of the bushing. Teflon tape or sealant should not be needed to seal.  Feed some clean ATF into the air inlet before you attach the air supply. When you attach air there should be no air passing or any tool operation.  Now pulling the trigger and releasing it should operate the tool and shut it off as normal operation occurs. If you have leakage or passing air the seat may be damaged during installation. There is no o'ring under the tip valve seat or on top of it by design of the Snap on product engineers. This throttle valve repair kit is my intellectual property and my design unique to only ebay and nowhere else.