I agree with the others. I'm one of the regulars over at ytmag N forum.. I also have a 8n, 2n, and a NAA ( same as jubilee ). The others gave you good info.
My first guess is that the rubber oring/leather backup washer is leaking.. You may need to hone the cylinder a tad and put new oring/backup washer in. One rare occasion the relief valve dies.. and on even rareer occasion the unloading valve dies.. but this usually doesn't result in a weak lift.
Sometimes on the piston pump, pieces of a dead piston oring/washer can hang up in the valving and cause fast leak downs.. etc.
Lots of ways to procede.. If the vane pump died.. a piston is probably your only replacement.. there was 1 2.5 gpm pump for the NAA and a 4gpm one for the hundred series... both bolt up the same.. but the 4gpm one ~may~ cause your draft hydraulics to be over sensitive... if you don't use draft mode.. it wil be faster hydraulic lift..etc. Either of the piston pumps will need the new manifold set for the hydraulic intake/exhaust lines.. Not cheap either way.. ( I hope it is a 10 dollar o-ring/washer/honeing fix.. )
Refill with NH 134d oil, or universal transdraulic oil...
Soundguy