Sealing between cylinder tube and cylinder head
An other reason for leakage that seemingly takes place via the rod seal, may instead be a failing sealing function between the cylinder tube and the cylinder head. This can be caused by an insufficient deformation of the O-ring or too small a section of the O-ring. The use of O-ring materials of low quality with low compression set value can result in a substantial permanent deformation, resulting in leakage.
A bad surface roughness value of the the seal housing groove or too large a fitting clearance between the cylinder tube and the cylinder head are other possible reasons for leakage.
Back-up rings should always be used together with an O-ring for static sealing between the cylinder tube and the cylinder head.