The O-rings for this have been updated. I get mine tight, even on the small 3mm allen bolts. I oil the o-rings and both sides of the fittings. Then pull it together very slowly 1 turn at a time on each bolt alternating.
This will give even pressure on both sides of where the o-rings contact the valve.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS