As Tim H says, there's no point in pumping grease down from the top of the kingpin - it is simply a longer route to getting grease to the same point as the stubaxle grease nipple (zerk).
I use the top grease nipple to pump EP90 into the stubaxle chamber. It ensures that the kingpin and bearings are always lubricated as the vertical and twisting motion of the axle, particularly whilst cornering will 'wipe' the grease from the contact surfaces.
This was Dave Rutherford's approach to lubricating the kingpins and as the first, I believe, to start making hard-chrome kingpins for Morgans, and a well qualified engineer, this is my adopted approach.
