I was chatting to a fellow driver yesterday and I was asked if I remembered the blue auxillary airline.
I did but havent had a truck with one fitted since probably the mid 80s.
I cant remember what they actually did, was it to do with the handbrake/deadman ?
Weve still got a couple of old trailers with the connector on but does anyone out there still use them ?
You’re correct, the blue line was for the deadman/trailer brake, it was common, on mostly British lorries, until the introduction of ABS, then it wasn’t compatible anymore & disappeared, it was a PITA if you were pulling trailers from the ports as the continentals used a two line system & a lot of their trailers only had two fittings, meaning you had nowhere to put the blue line & a GV9 if you were caught with it not connected, there was a valve you could fit that fed into the yellow line so that you could stay legal (& have a seperate trailer brake)
Cheers newmercman 
Dont suppose anyones still using them now then except for maybe the restoration brigade
They were fine if you had a regular trailer and the brakes were OK. In winter going downhill you could just hang the brakes on the back trailer wheels a little bit so they were just slowing the back wheels down and drive the rig slowly downhill. Not recommended if brakes not working OK as it could throw the back end into a skid.
As was said above the blue line was for the deadman, and I have been issued with GV9’s
(PG9’s for your young 'uns) for pulling a two air line trailer with a unit fitted with three.
The valve didn’t need to fitted to the truck, but would be ‘loose’ - it had ‘c’ coupings so that
the yellow and blue air lines could be fitted at one end, and the other end had a coupling
to fix it to the yellow line on the trailer. From memory it was about six inches long and
enabled the deadman to operate via the yellow line.
Just one of the little gizmos we had during the '70s and '80s to pull foreign trailers, like
an electrics converter (seven pin socket on 12 inches of flex with a five pin continental
plug on the other) and a box coupling (some trailers notably Italian) had no palms or
‘c’ couplings but a square box with two valves in it. Then you needed the corresponding
half of the valve with two lengths of hose and palm or ‘c’ couplings on the other end.
Not to mention a box of various lenses, bulbs and various trailer handles.
K