Verify that all of the glow plugs are working properly, and make sure the fuel filters aren't clogged...

I know the primer pump isn't supposed to leak, but I'm not really sure how much of a a problem that would pose...or if it would even affect the car's starting... :\

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Current cars:
2000 ML55 AMG, 174k miles
2003 C240 T-Modell, 202k miles
1995 S320, 207k Miles