I just did that on my 85 300SD 126 for the longest time I was fooled into thinking it was leaking at the return line where it goes into trans. until I really looked close as someone was adding trans fluid I was watching and saw where it leaked. Fairly easy job of taking dipstick tube off and putting new o-ring in there and also I made a gasket at the base and put that in there as extra measure as there was no gasket there and I thought there should be finally no more trans leaks. That one can be a real fooler to locate.