I remember the Scania 142 had 2 levers, one was the handbrake and the other was the trailer brake. Right next to each other on the dash.
Iv drove a few 164 series that had the separate trailer brake, great piece of kit.
I remember talking with my dad about older tractor units and the first scania he ever drove came up, separate trailer brake leaver in the cab reserved for people right up your arse. Give your trailer brake a quick 2 second pull and you’ll guarantee brown underpants of the offending motorist. Or so he said 
TheYoungTrucker:
Iv drove a few 164 series that had the separate trailer brake, great piece of kit.
I thought they stopped fitting them on the last 3series are you sure that wasn’t the retarder
Had secondary brake on F86s. Handy for keeping home made A frame in line 
Beau Nydel:
Had secondary brake on F86s. Handy for keeping home made A frame in line
And heres me thinking F7s are old 
Trickydick:
Beau Nydel:
Had secondary brake on F86s. Handy for keeping home made A frame in line
And heres me thinking F7s are old
Unfortunately it’s not just the trucks!
Remember first time in a F7 thinking god things are moving on!
I have an MOT booked later this month for a three line air trailer. It’s the best one on the fleet, it’s certainly had less money spent on it over the last 18 months…and three of them are almost new , one of these is up for 1st test the week before. It will be time for the test station to get out the adapter to check the secondary brake operation.
Trickydick:
Blue line is a secondary brake and it was a proper seperate brake system with its own relay valve on the trailer and two air pipes to each brake chamber, one for std air brake and second for secondary brakes, secondary brake were not load sensed so were full bore loaded or empty, this was before spring brake chambers which work differently.
Later on the two trailer brake lines (blue and yellow suzies) went to a valve called a shuttle valve which then sent either air signal to the load sensed relay valve, then they did away with the secindary bit altogether.
The standard air line couplings are called C couplings and the blue line ones were called CA couplings.
And if your 3-line tractor was pulling a 2-line trailer, you couple the blue line to the “fourth line” back into the tractor - I have this set up on my Atkinson
Even earlier trailers were 2 line, before the 3-line system was introduced, so “new” 3-line tractors were required to pull older 2-line trailers.
I remember in the days when suzies had taps, if you put your red line in the tank along with a pipe, covered the top of the tank with a rag to stop the air escaping, put the other end of the pipe in another tank, of another truck, switched the tap on and kept the air built up, you could transfer diesel across to another truck. 
A useless piece of info …or did I just dream it?
I remember the blue line from the sixties. Best thing in it’s day, if it was used properly. I can’t remember the firm, but one mob had a sign in the cab saying not to use it as a parking brake… I wonder why. 