Actually I have seen professional techs have problems in this area. What I do is soak the o-rings in oil then install them. Next put a little drop of oil on the mating surfaces of the expansion valve before installation.
When you install, you will have to tighten the 3mm allens side by side at 1 turn each, alternating untill done. Then install the outboard pipes carefully making sure that they are fully engauged. Then tighten.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS