I have 2 Eheim Pro II 2026. One has been running for over 6 years and having co2 pushed through it. This year the same problem began for me. Running it is sealed fine, but if I turn it off water begins seeping out the back.
I have tried everything including replacing the sealing gasket and changing up the piece that connects the hoses to the filter and putting vaseline on the seal- with no luck.
The solution is simple. When I need to turn off the power during weekly prefilter rinsing, I just open the shut off/disconnect to stop the water coming from the tank. This prevents the leaking. When done I close it back up and plug it back in.
The problem only happens on one of the two filters.