Personally i saw bugger all wrong with the proper 3 line systems, one fails there’s a very good chance you’ve still got a full secondary system via the dead man or initial pull on the parking brake, depending on the make, via the blue line.
For newer drivers, our lorries used to have 3 line braking systems instead of 2, the third being blue, it was fully independently piped right through and down the trailer, and operated on some old stuff via the dead mans handle, which was usually a lever of some sort which operated (though happy to be corrected as there were many variations) on the front axle of the tractor and the trailer brakes only…in the days before ABS it was an emergency brake that in the event of a jack knife starting (drive axle locking up), you could, in theory at least, release the footbrake and apply the rest of the brakes, hopefully to straighten it up again and stop it straight, in practice a real jack knife is sudden and terrifying to the poor blighter and it would take nerves and instant reactions to do this.
It was also a secondary system in case the yellow line system failed for any reason.
Notably some Scanias in particular you also has a large lever which acted on the trailer brakes alone as well, this needed to be treated with great respect, for many vehicles had no load sensing brake variance and the trailer brakes were instant on lock up, but for the owner driver very useful (cheap) on traction work cos you could use the trailer to do all the braking.
I’m almost certain that the initial pull on some lorries without a dead mans handle, the parking brake operated this system the same way (some designs, later Fodens for example, the secondary brake was a stright pull in the parking brake, and to engage the park brake fully you eased the park brake lever round a gate of some sort, other makes there would be a release handle incorporated in the park brake lever), but again happy to be put right if i’m wrong her, often am…
My first artic, a mickey mouse Foden, as with other lorries of the era, didn’t have an air operated parking brake at all, it had a transmission drum brake like a Landrover but in the Foden case was a drum fitted to the back of the diff, these designs obviously would have a dead man as the park brake is effectively useless as an emergency brake…wasn’t much better than useless as a park brake come to think of it at all, and for hill starts etc you’d use the dead man just as you might use hold your new lorry on partial park brake for smooth hill starts.
What i should have mentioned, is that these old vehicles didn’t have spring brakes, and when the air drained out the brakes released themselves, so even though it was safer to leave your vehicle with the dead man supplementing the drum park brake, in practice after about an hour the air would have drained out and the air brakes released themselves…the trailer parking brake of the time was a ratcheted cable which was invariably seized solid through lack of use…ahh the good old days… 
Never liked the knowledge that a service line failure could leave you with no trailer braking at all on a 2 line system, hence just another one of the many reasons for utilising auxilliary braking as much as possible in order to keep all brakes as cool and understressed as possible in case they’re needed desperately, i’m quite pleased to learn from Bking about this electronic trailer braking in the event of no service line for some reason, would like to learn more about it please, but in lorry driver, not too technical 
,language.
No one was hurt etc, but some other driver’s pride this time took critical damage! 
