My bus is a 1995. All of the air lines are DOT plastic. The only exception is they used flexible air hoses to the brake chambers instead of DOT plastic line. There are bulkhead fittings that convert from the plastic to threaded connections. I had the tag axle flexible hoses replaced in 2011 and they haven't leaked yet.
OK, I spent many years writing parts manuals and specification lists. Apologies if this is too pedantic, but I have three kinds of brake piping on my bus:
1) Solid stainless metal pipe (mostly around the brake pedal/valve);
2) Poly-plastic "DOT" tube, Conforms to SAE J844. (Note - this is solid-wall tube, it is not reinforced by woven fabric -- it is "tube" not "hose".)
3) Brake hose, also "DOT" approved; with crimped on metal attachment fittings and the flexible wall reinforced by woven fabric- that woven fabric is what makes it "hose".
Don, it sounds as if you have asked about genuine "hose" assemblies. If you have one that's leaking, it was just made incorrectly. I'd try to return it but even if you have to buy a new one, they're cheap. My hoses (non-compounding relay valve to brake canisters on the rear axles) needed to be 22" (2), 20" and 19" to fit well but a 22" will fit OK at all the points if the extra is secured with zip-ties so I carry one of those for a spare. Fronts are 28" so I carry another spare the right size for those. The hydraulic parts place down the road from me said that he could make hose assemblies that could be disassembled and put back together but that's *way* more expensive than the non-disassemblable kind so I just used the NAPA hose.
Chessie, I'm not saying you're wrong but if anybody was to be found running plastic DOT pipe to the brake canisters around here, they'd be off the road until flexible hose assemblies were installed (the picture in your mind should be of a guy with a grey uniform wearing a smokey-bear hat standing there with a ticket book in his hand watching the work being done). I'm not saying that plastic tube wouldn't work, not saying that it's not commonly done in some places, not saying it's not safe, not saying that the Federales won't let them go -- but you won't find solid tubing used on the brake canisters on my bus.