Ther is an o-ring under the spring and metal washer, there's also an o-ring at the top of the tube where it meets the inlet manifold.
Hopefully it's leakin from the top o-ring as this is much easier to replace then the bottom one.
To replace the top o-ring slide the tube down against the spring pressure and flirt out the o-ring.
To replace the bottom o-ring you'll need to remove the inlet manifold so you can pull the tube out
Cheers,
Iain T