Remove the trigger by pressing the roll pin out at the top of the trigger. Lay the gun on its right side and use a small metal punch and ball peen hammer to tap the pin out and remove the trigger. Remove the roll pin in the handle, directly behind the trigger, in the same way.
Withdraw the trigger valve from the handle and remove the black rubber band seals around the valve. There are two. Replace them with new seals of the same size, fitting them into the grooves on the valve body. Fit the valve back into the handle and press it into the handle.
Turn the gun over to its left side. Align the trigger with the hole vacated by the roll pin. Press the two roll pins back into place, tapping them lightly with the metal punch and hammer.
Remove the four screws from the square black cover on the back of the gun, above the handle. Use an allen key to turn the screws counterclockwise to loosen and remove them. Lift the cover from the tool. Push a small metal rod through the barrel of the tool and press the driver cylinder out the back of the tool. Remove the black rubber O ring seals and replace them as you did with the trigger valve.
Press the driver cylinder back into the tool and push it all the way forward to seat the O rings. Replace the cover. Install the four screws and tighten them with the allen key.