Slow start

So I've had a slow start ever since I've owned my 300tdi only once it started right up then never again haha any way can a knackered injector pump suck air into the system? I've got a piece of clear tube from filter to injector pump and when I switch engine off I can see a few bubbles coming from injector pump back towards filter. I will check tomorrow morning on startup and see if any bubbles come from filter or if there is any fuel at all in the pipe. Bleed of pipes are good glow plugs working good I've got new pipe from sender to filter with inline fuelpump. It's a edc also I've got a leak out the top of the injector pump. Also it's got the original sender unit from factory no visible rust. Could my sender have a tiny hole? Cheers

Also I have yet to check timing of the pump and the timing belt if it's jumped a tooth cheers

Before looking at fuel issues, make sure engine/gearbox return earth cables and connections are in tiptop condition, no rust. This is primary cause of starting issues.

+ 80% of fuel problems are electrical, and 80% of electrical start problems are fuel

Try a short pipe with fuel can direct to pump and ensure 12 v at fuel solenoid with ign on