If theres a large enough restriction in the return, it will cause the front seal to leak, In my case, once i fixed the issue of the return, i have the same cp3 in there and its not leaking anymore.
Are you 100% sure its the actual cp3 front seal thats leaking? have any pics of where the fuel is leaking to? check all fittings, hard lines etc? hard lines om good condition? use leak dye to make sure where the leaks coming from?
did you just clean out the sending units or replace them? did you pop out and check the little fitting in the bottom? pics in post #38 and #42 of this thread.