Ah the old blue line secondary system.
Back in those days you needed a decent park brake because the brakes weren’t spring brakes, when the air drained out the brake released, my first artic being an S39 Foden plus York 40 ft tandem axle flat trailer, a brake drum spinning merrily away on the back of the diff comprised the vehicle park brake, bearing in mind how a diff works in practice this could mean only one wheel was preventing 32 tons fro rolling away once the air drained out.
The blue line secondary system operated on the tractor unit front wheels and the trailer wheels only, there was a pull handle known as the dead man mounted on the casting in front of you amusingly know as the dashboard, for normal driving you used the dead man as a holding brake on hill starts etc because the diff drum brake was to all intents and purposes utterly useless, what you had to remember though was even though the dead man would stay in the position you applied it after a while the air would drain out and the vehicle roll away, so remembering that even though the handbrake was dire you still had to apply it, even if you discovered on return to the vehicle if parked on a damp hill that it often wasn’t where you left it because friction or lack of it would allow the drive axle tyres to slip a little one side then the other.
Course anyone sensible parking on a hill would apply the trailer park brake, oh yes a ratchet spool of steel cable which if it wasn’t seized solid exposed as it was to all the road filth some clot had probably wound the thing on with the cable off the spool which would prove impossible to dig out and required cutting off and replacing.
The reason for the blue line not operating on the drive axle was in the hope that if the vehicle started to jack knife (typically drive axle lock up under braking) then if there was time the drive could release the footbrake allow the vehicle to straighten up and then use the dead man.
In practice a jack knife is a violent sudden event, the chances of the above theory working for you were probably in the lap of the gods.
Remember in those days almost no vehicles, semi trailer in particular, had load sensing brake proportioning valves, when you applied the foot brake you got full brake pressure hence the often seen full smoke outs of locked trailer brakes…the canny driver if the brakes were too harsh might slacken off the (all manual) adjustment on one of the rear axle brakes so only one axle would lock up prematurely allowing hopefully the unlocked wheels turning to keep the whole lot fron slewing sideways, nasty dangerous events they were.
Overall i liked the blue line fully secondary system, in the event of a yellow line burst you still had full trailer braking, but a proper park brake such as we now have would have been a blessing given the pack of spring brake chambers.
Someone mentioned the second brake lever above, on Scanias (i drove 110/111 with this fitment, can’t rebember if 112 still had the trailer brake lever), the overly long brake lever on the dash operated the trailer brake only
As for applying the park brake manually on modern trailer, auto or not, its barely necessary though keeps box tickers happy, BUT, recently i had a trailer move away when trying to pick it up (no i hadn’t checked it was on before attempting to couple, but you ask any tug driver if he does that) only to find some brain donor had pushed in the shunt valve, effectively releasing the brakes 
