The white seat goes in the bottom of the freshly cleaned tool handle bore first. There is a white raised circle or 360 ring on the white seat that must face up toward you. The circle ring is where the face of the red 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.  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 red/orange tip valve stem into the trigger stem hole in the trigger stem through the white 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 red tip valve nub. Change the black 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. One customer told me he had to put sealant on the side of the seat to seal it with the tool handle bore. There is no o'ring under the tip valve seat or on top of it by design of the Snap on product engineers. 

The 2 smaller o'rings in my kits are for the IM5100 reverse valve body seal. The OEM reverse valve o'ring leaks over time and my recommendation is to use 2 side by side in place of the single o'ring that comes with the tool. The larger o'rings in this kit will fit the steel air inlet bushing for the tool. This throttle valve repair kit is my intellectual property and my design unique to only ebay and nowhere else.