For years all that was necessary was a pair of new o-rings. In the last five or so years the replacement R4 has used a seal with a steel washer and rubber bonded to it. Since this washer arangement came out I have seen 2-3 manifolds that have been overtightened to the point where they are no longer staight across.
There are now at least three different seals for R4 compressors: o-ring, thin metal washer/rubber, and thick metal washer/rubber.
I have not decided when the two different washer seals are used, but the compressor seal area is different for the o-rings.
Bosch Master, ASE Master, L1
33 years MB technician