We have two of these. They are a lifesaver when sniffing out hydraulic leaks. We connect a propane tank to the tube in question, and start walking. Alot of times you can hear the gas blowing out of the leak before you sniff it.
For your main hydraulic pipe, I would think you could plumb in a tank at the source, shut off the connections at all satellites, then start walking. These things are pretty sensitive, and as long as you know the general area, you should be able to find it pretty quick.
They had one here when I got here, but when it broke, the labor savings alone was worth justifying a new one. We can find leaks in under 10 minutes with these things.
https://www.grainger.com/product/EXTECH-Combustible-Gas-Detector-2ELP8?s_pp=false&picUrl=//static.grainger.com/rp/s/is/image/Grainger/2ELP8_AS01?$smthumb$Keith, you can always borrow mine!