I would take your back latch/back cap off and replace all the orings in that back latch/cap area. It's much faster then swaping out every o-ring to find the cause.
The latch shaft's 2 = 1x2.5 orings
The inner back cap o-ring = 17/70
The bolt guides rear = 15/90 (unscrew the bolt guide to change this one)
The latch o-ring = 16/70
Link to the o-ring chart