Maybe a mention in the instruction manual about that fitting would be helpful. Of course, you are probably swapping springs out again, so it is on its 3rd or 4th re-torque and the manual hasn't been consulted since the initial install, so that wouldn't help in this case.

About 8 to 10 ft lbs of torque, if that, is all that fitting is designed to need to make a seal. It is a very soft brass sealing surface and is meant to be smashed against the nipple to make a seal. Ask it to take 50 ft lbs of torque so it doesn’t leak and that is what happens every time. That fitting should be accompanied with a tag that says "WARNING DO NOT OVER TORQUE WITH UNCALIBRATED MAN-HANDS" :-laf