Was talking bowlocks with some colleagues, as you do while waiting for a run, and the conversation worked round to the 3rd/blue/auxiliary air line from days gone by. We couldn’t agree on what it did/served. 
Can any one enlighten us? 
If I recall it ave enough air for brakes if red line inoperative
I’m sure someone will be along shortly to put me right
I know yellow line was signal pressure to brakes, help me out someone I’m diggin a hole here !
Jim
The blue line was the secondary system, often referred to as a deadman. On a two line braking system it’s piped into the service line, you used to be able to get a valve to plug the blue susie into so you could pull two line trailers with a three line unit and this is basically what happens now on a two line system, the function is still there, you just can’t see it anymore 
It was to feed red diesel to the engine from the secret tank which was hidden in the trailer bulkhead. 
As Newmermacman rightly points out.
Mind you should the unthinkable happen, and the yellow line breaks anywhere its piped, then the current secondary system is about as much use a chocolate teapot.
Bugger all wrong with the full three line system, it wasn’t broke but the sods weren’t happy till they fixed it.
Harry Monk:
It was to feed red diesel to the engine from the secret tank which was hidden in the trailer bulkhead.
Yeah thats what an old employer of minehad till it was found but he did get away with it for a few years, cost him quite a few thousand to pay the duty back
newmercman:
The blue line was the secondary system, often referred to as a deadman. On a two line braking system it’s piped into the service line, you used to be able to get a valve to plug the blue susie into so you could pull two line trailers with a three line unit and this is basically what happens now on a two line system, the function is still there, you just can’t see it anymore
I still maintain a three line trailer for a customer, so I have to keep an adapter made up for it. The manufacturers keep altering things so assuming things work in a certain way isn’t necessarily what you are going to find when you look more closely. For instance you won’t find a relay emergency valve on a lot of more modern trailers, the emergency feature is built into the park/shunt valve now and not all of those available necessarily work the way expected.
yup Harry got that right …lol was the first thing i thought off 
.