There are lubricated O-ring seals between the moving parts. The elastic O-ring is seated in a precisely machined grove and slightly compressed during assembley. There should be no nicks, burrs, or scratches at the metal surfaces over which the O-Ring moves. The gap between stationary and rotating parts is filled by the O-ring. The gas pressure should move the O-ring closer to the gap. 

See the images at the bottom of page 6 and 9 of this [O-ring handbook](https://www.physics.harvard.edu/uploads/files/machineshop/epm_oring_handbook.pdf) PDF.