So if you've replaced the washer or o ring. There has to be one or the other in that fitting. First thing put some Teflon tape on those threads. Along with any other hydraulic fittings you take loose. Make sure you hold the tape back from the ends a few threads of course.
If you still have a leak replace the fitting. You'll have to pull that line and take it to someone who builds hydraulic lines.... They wear out especially on something like a post driver.