Hi, I have a mk2 focus that’s a pain to start but fine once running. 
 

so after having trouble starting popped the bonnet after I finally got it running and noticed a few air bubbles in the fuel lines after the filter, had the filter and housing replaced as this seemed the logical thing to do since bypassing the filter and connecting both in and out together cured it, no bubbles or anything and ran fine- no luck, how could it run fine without the filter connected but drag air in and through while on tick over when connected? Doesn’t like to start could take 20-30s of cranking before finally going but air is pulled back in the pipes. 

any help or advise would be great thanks 👍🏻

I am going the same route (S-Max, no lift pump)...
Tested with Mityvac from return pipe to filter inlet, would not hold vacuum.

Across filter housing, dropped identical amount therefore filter housing or connections is faulty???
Waiting for new to arrive.
Your issue seems similar so you must have bridged filter with new connections, so only thing is that your bridge is a good seal where existing pipes on new housing isn't - sorry, that's all I can think of!
I sealed pipes with Vaseline and no bubbles in line from filter to pump for 2 days then fault returned.