After making many dozens of this type of control valve, maybe I could give you a couple of pointers that 'might' give you a leak free valve.
If possible, lap the two faces together so that you get a nice flat looking face. You will see after you start where all the high spots are, and you keep going until you have a complete dull surface on both parts. Don't try to use power tools, your fingers are more sensitive when the correct surface finish is reached.
The spring loaded part is one area that is affected by vibration and your inputted rotation, so I would suggest you make the centre spindle a little longer so that you can use a locking nut on top of the nut that is on there, or make two very thin nuts so that they can be fitted into the space you have. I always tried to finish it off by using an acorn nut to do the locking, it looks much neater than just two nuts and are very easy to make.
This shows the locknut required
And these are the acorn nuts, you make them to whatever length is required.