Newbie fuel leak help on 1996 E300D

[clydecpa]

Greetings to all, new member here. Long time diesel owner but first Benz.

Well, my '96 E300D with 137k miles has a fuel leak (or seal problem) that I can't quite seem to find, or I should say my mechanic can't find. They are an independent shop, good people, experienced, have all the Mercedes equipment to do the job well.

Thusfar, we have replaced the o-rings (at or below the injectors?), fuel return line (had a bad visible leak), and the main fuel filter (bad seal). All clear fuel supply lines were replaced within the past 18 months. She's doing better now, but if I park her on an incline overnight it takes a long time to prime her and get started the next morning. There are significant air bubbles visible in the clear fuel lines going into and out of the fuel filter. If I back into my driveway, no problem starting the next morning whatsoever.

I assume that since this is the case, and that once started she runs fine, that my fuel pump is probably fine, no? But, that is what the mechanic suggested as the next most likely culprit. Is there some type of valve that if compromised, could be letting fuel drain back into the tank when parked on an incline? I have been told there is also a pre-filter on these cars, what about that? Sorry, I am no gear head. I change my own oil and oil and air filters, that's about it. Hey, it puts me ahead of 95% of the population, doesn't it? Also, now there is no visible sign of fuel leakage (driveway, under engine, smell of diesel), just unwanted air getting in.

Any words of wisdom?