172 Nosewheel Struts Leak Nitrogen

I rebuilt one and the best A&P on the field rebuilt the other one, on 1966
and 1957 172's. Neither will hold nitrogen. What's the secret to getting
them not to leak.

I bought a complete O-ring kit from Trimcraft in Wisconsin. My personal
A&P told me that only one or two large O-rings were leak-prone and crucial
for replacement so I replaced those only, at his urging. Now I think I'll
install the complete kit, which includes at least 6 O-rings. How about it
A&P's who are Cessna savvy? What's the trick here to getting these struts
to stay up and not leak nitrogen? I know they are not leaking around the
Shrader valves. I have done too much testing with soap suds looking for a
bubble to emerge. Besides, both of the Shrader valves and the valve stems
have been replaced and are official Cessna parts.

The flight school A&P on the field says some of the struts just have to be
re-gassed with nitrogen occasionally, but that's not a satisfactory answer,
far as any owner would be concerned.

Thanks in advance for any good advice you can give on this frustrating
problem.