300 tdi engine start very tricky

Hi Lads. I have question what do you recommend testing trying to fix starting problem of my Def 300tdi. It started say 5 months ago out of nowhere. I couldn’t start the engine within 2-3 seconds as usual after glow plug went off I twisted the ignition key and it just kept turning the starter motor but engine did not start. So I taught it may be the winter conditions /before I had no problems/ so I took the battery out and tested it and just to make sure I put it on charge everything was fine with battery /8 months old 140Ah, also checked the alternator and it gives a steady 14,4V when engine is running/ after fitting battery back tried to press gas pedal for like 20 times and engine eventually started up after 6-7 seconds of cranking. Then my next step was to put full tank of diesel fuel with winter additive and also got start spray for air intake with ethanol to help with cold starts//great stuff/. Alright then I searched internet because problem still persisted /start spray into air intake really helped a lot/. And I found that earth strap cable may have wrong connection with chassis and gearbox, so I checked that out cleaned battery terminals, cleaned connection points and bolts on chassis and gearbox and greased them with copper grease. After that problem is still there. When I left the car sitting for couple of days I had to pump gas pedal at least 30-40 times and let the glow plug be on at least twice then it starts, after a ride short or long and I park the car it starts like new without problems all day. Please can you give me your opinion?. How do I check glow plugs or I do not know if the injectors or anything else could be the problem. And i nearly forgot i have not fitted any imobiliser or anything like that. Thank you

most likely fuel system quite possibly air entering through poor leak off pipes, poor lift pump allowing fuel to drain back and give poor pumping

Thank you, I will replace fuel lift pump and check the lines for any loose connections.

check your injector leak off pipes-they perish

I'd change the fuel lift pump,and the fuel filter as a precaution