I have a '80 300TD and I have been experiencing poor to very poor idle when the tank is less than 1/4 full. The rest of the time it idles just fine. I have also been experiencing fuel leaking from the injector bypass lines and around the black plastic ring around a couple of injectors after prolong driving. Any suggestions?

You need to replace the braided bypass lines and check all injector connections...leaks are not good and should repaired. I would also change the fuel filters. Maybe you have some crap in your fuel tank or the pick-up in the tank has a problem.

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1983 300SD 343K everyday car
1983 300SD 285K from junk yard-tooks parts from deer car- runs great. Brothers car.
1984 300SD parts car-Hit deer
1979 300D 175K non-turbo "Doctor"
1979 300d parts car

regarding the idle and fuel tank level, read about my experience:

http://www.peachparts.com/shopforum/showthread.php?threadid=48396

short answer to the above question: check your fuel tank screen (in addition to what CHAPP said...)