I don't have an answer for you. However,I can tell you that I've had a bummer of a time trying to get my XP3 to prime after I installed a CO2 reactor on the output side. I don't have a gas bleedoff valve on my reactor, which would probably help. I think the reactor fills with air from the filter as it starts to siphon water down the input line. The air in the reactor can't seem to build up enough pressure to push water up the output line. Once the pressure builds high enough, it prevents the XP3 from siphoning any more water. I think if I had a bleed valve on the reactor, I could open it and let the air escape from the output side of the filter, and everything would be ok. Probably a newbie error on my behalf!
My first DIY Planted Tank:
4x & 2x ODNO Flourescent Lighting (2-6500k, 1-5000k, 1 P&A)
Homebuilt Pressurized CO2 Injection
Rena XP3 Filtration
Still using my original gravel substrate