I would be asking myself why has the nitrogen pressure dropped. My guess would be a seal has gone, does it have any oil in it? Something is leaking somewhere either oil or gas.

Either way I personally wouldnt use it without a proper strip and rebuild, just regassing it isnt the way to go in my opinion.

That end cap is neat Andy, looks like the one on my f3 shock. Those rebuilt shocks look superb, I think Im gonna have to do a full refurb on my 28SP shock now Ive seen those