We have a cylinder and the nipple for one of the hoses has a hairline crack and dripping, we are worried if we tighten the hose more going into it, it will open the crack more. Removing the whole cylinder to have a new nipple welded on it a rather larger under taking. Brazing hold up under pressure? Repairing while on machine would contaminate hydraulics?
It's hard to tell without a photo, but generally speaking trying to do any sort of weld repair (brazing included) on a hydraulic cylinder in service is a waste of time. Without thorough cleaning inside and out, the oil will always bubble up through the crack making a successful repair impossible. Take it off, fix it once, fix it right is my advice.
You should be able to screw a nipple and coupling or a street L into the cracked fitting and then weld both the crack and around the nipple. You should be able to avoid contamination this way by not welding to the barrel. You still need to get the oil away. Take your time, use a small rod and weld in short spots letting it cool between. It has worked before sucessfully.