Check the little rubber piston seat as well. This caused a leak on my DM4. I pulled it out and a small piece of it was chipped which led to a really annoying sound. Swapped it out and it sealed like a charm.
DM8 - Virtue OLED/Laser eyes, BWing Bolt, CP On/Off, CP Flow Plug, CP Trigger, Hyper3, DM9 Eye Pipe, UL Barrel, Rotor
DM4 - CP On/Off , CP Flow Plug, CP Trigger, Ramping Chip, UL Barrel, Empire B