It’s common knowledge that any car worth owning guzzles an ancient fuel called diesel.
Diesel is cheap but generally, old diesel cars are also pretty bad for the environment.
The composition of diesel has changed over the years and the seals in diesel pumps don’t like this. So eventually they just give up on life and this is the point where a diesel-car-owner also thinks about giving up on his. How to diagnose a dying fuel pump seal:

• Your mileage goes down significantly.
• Your car smells like a diesel factory.
• When you drive away from your parking spot you leave a puddle of excellent good fuel behind.

So diesel pump seals aren’t meant to last forever, and if the production date of this pump in question is about 18 years back on the calendar it’s not a big surprise that the Diesel in this pump does not care to be stopped by this 18 year old seal.

Also, Bosch likes it better when they get paid to repair their own equipment so they invented a stupid socket so you can’t easily open their pumps. Solution: make your own, anti-tamper-socket-tool.

When daytime is reserved for work, car repairs have to be done in the depths of night, luckily people invented flash lights and tiny mirrors on sticks.