Passat B6 1.9 TDI: Starting Problems & Solutions

Hey everyone.

I'm having a frustrating problem with my B6 1.9Tdi (BLS).

I often experience starting problems, sometimes after a long period of inactivity and sometimes after a very short one.

"Starting problems mean that I have to crank the engine for a longer time or more frequently before it receives fuel and starts."
I initially thought the PD element was leaking, so I replaced all the seals. The problem still persists. I replaced the diesel filter, and then I felt like it was getting better (the filter was also quite old).
After a certain amount of time, I started having the same problem with the engine not starting again. The temperature didn't seem to matter; it wouldn't start at -10 degrees Celsius, just like it wouldn't start at +20 degrees Celsius.
I also replaced the glow plugs, then the tandem pump, and finally, I installed a check valve on the return line from the filter because I noticed that every time the engine was higher than the tank, the problem got worse.

"I left it on my ramps overnight after installing the valve yesterday, and tested it again this morning. It still took a long time to start. Once it's running, though, and I use it daily, I have no starting problems, and it runs perfectly."

Lately, I've been traveling for work, and my B6 has been parked for two weeks at a time (relatively level), and even then, it started poorly, but not as badly as this morning when I parked it "tilted."

Based on the symptoms, it seems like the lines are running dry when the engine is higher than the tank, but where is the source of the problem?

Probably, I'm too late with the answer .

Option 1: The PDE is internally leaking/worn out.
Option 2: The PDE (power distribution element) has been re-sealed, but there are already signs of leakage around the head, and the sealing is no longer effective.

Would spontaneously come to mind.