If it is truly hydraulic line it will carry significant current.
Hydraulic lines have steel braid inside to contain the high pressures involved.
Bob, true... But will the carry that fault current safely? Even if they have steel braiding, they must have sufficiant contact with the fittings. If they were to take a hit from any circuit, the fitting and braid contact if any, would be the weakest link, and under considerable pressure.
However I was thinking more along the lines of equal potential in a shop setting. If not from the compressor itself, which have a tendancy to shake all of thier parts loose, maybe another circuit. Just using my imagination here: If this shop is (Even in a home) big enough to have piped compressed air, then its big enough for other large equipment, like a welder... Who knows? Oh ahultin does.... Whats the shop for big guy?
Anyway, the hose could be removed, abandoned, whatever and you would still have a large metal conductor running around carrying voltage.
Now watch this... This is where we debate the nature of the code...."The bonding jumper(s) shall be sized in accordance with 250.122 using the rating of the circuit that may energize the piping system(s)."
It says circuit that may energized the pipe, that doesnt nessesarily mean the compressor. If you have a 1/2" or larger pipe running around the room, that could mean any equipment in the room. That compressed air could be connected to another, or multiple machines of higher amperage.
I recently worked on a cabinet shop that had several machines like automated mortice machines, that had pnmematic automation and sizable 3 phase 480 motors. Oh wait, my friendly glass guy has a 200A behemoth of a glass shaper that eats 2" X 12'X whatever size glass, that also uses air from a 30A compressor that feeds the rest of the shop. (There the air is bonded to structural steel by common framing.) True they all have EGC's but would the hose that feeds them qualify as a bond?