if I understand you correctly, the leaking stops when the packing nut is tight, but leaks after it loosens. Leakless pumps use a variety of seals that do not crush with the packing nut, like packing does. Assuming you have an O ring or lipped neoprene seal behind the nut, I suspect the nut merely holds the seal in place. Suppose you put an adhesive sealant like RTV or a thick one like teflon pipe dope on the threads to hold the nut in place? Some leakless pumps have a ceramic or neoprene seal behind the rear bushing. I can't see how the packing nut would affect them. In the long run, or maybe even short, you are probably looking at getting a rebuilt pump.