I make my own pipe fittings and just turn up a blank cone when I want to block off an unused line. Soldering an offcut of brass or copper rod into the cone, or olive, might be a bit quicker I suppose.
Grant is likely thinking of wear in the chassis bearings and after 20 yrs in storage, oil could also dry out and turn gummy. It would probably be easier to support the loco upside down on the bench and run it on air, than fiddling around with an airline outside on the track.
As you did, it is always good practice to do a hydraulic test on a pressure vessel after any repairs, or modifications. It wouldn't be the first time a silver soldered joint opened up a pinhole when reheated.
My comment about compressors when testing on air was just putting numbers on why a chassis can sometimes run better on air than on steam. A workshop compressor is an unlimited supply of air for a small loco chassis, while model boilers can usually only supply just enough steam to successfully run the loco. when everything is working properly. I'm not sure if measuring air usage would tell you much, but it would be easier to do on the bench than on the track.
Graeme