After passing test and having it drummed into me about BLACK etc no one were i work uses the trailer brakes even the mechanic said no point they come on when you pull red line, So if this is the case why is it there lol seriously though why do they have both systems?
What would happen if you did not use the trailer brake and for some reason forgot to apply the unit handbrake 
Ha thought of that one as i posted it but it was the only scenario i could think of 
Its quicker to push the button than unclip the airline i suppose
I know a few older guys who never use the trailer break, they just take off the red airline and trust that. I have heard many stories of the trailer running away and destroying a few things though 
Hey all, its pretty simple really. On all modern air braked trailers have spring parking brakes. What this means is that when the red line (emergency line) is disconnected the park brake automatically engages. This is all down to the spring brake chambers, they are called this because when the air in the brake chambers is released a powerful spring applies the brakes. As your driving along the 125 psi in your trucks air brake system keeps the springs compressed and the brakes off.
The brakes can engage when your driving if the air pressure drops too low, and this is why air brakes are safer than hydraulic brakes, if the brakes won’t work because of a lack of air pressure then the park brake engages. Also, if you notice a brake binding on one wheel this may indicate a faulty/worn spring brake chamber slightly applying the brake, causing it to drag/heat up which obviously will need repair, quite a common fault.
Now, as for why you are told to use the park brake button on the trailer for the driving test. Its because not all trailers have spring parking brakes, old trailers have a ratchet cable system to pull on the brakes. So if you come across one of these trailers with a ratchet park brake, make sure you use it or it may roll away! But of course you are unlikely to use such old trailers. So all you need to do is pull off the red line, but for driving test you need to show that your capable of safely applying the parking brake on any trailer.
However, it is right that applying the park brake button makes coupling safer incase the driver is still half asleep. Imagine this; trailer parked without park brake button engaged but its held by the spring brakes because the red line is disconnected. Next driver backs up to the king pin and engages the trailer and exits the cab without engaging the cab handbrake. Driver jumps out, winds up the legs and put the red line on. Then what happens - just as the spring brakes automatically come on they also release when the red line is connected. So then it happens, the vehicle begins to move and where is you the driver? On the catwalk and may possibly get crushed, not good i hope you agree.
So remember, just pull out your red line to park but not on ratchet systems and always always put on your cab handbrake. Its happened to me before, as soon as the red line goes on the vehicle starts to move, if this happens don’t panic, if your on the catwalk then just pull out the red line to bring the brake back on.
If your lucky enough to have a modern tractor unit they may be a warning buzzer that sounds if you exit the vehicle without applying the brake.
Many thanks for above post very interesting 
Worth noting also that once I thought I could trust the trailer without the parking break on and some idiot from the warehouse had tried to move the trailer while I wasnt there and pushed the shunt button in so as I approached to lock the pin the whole trailer started moving because my unit lifted the trailer legs enough to so they were off the floor and the air brakes were not on as the shunt button releases them 
For the sake of damaging a person or equipement is it really such a hardship to get out and just check it is on or indeed just turn them on
without going into detail i would say its just good practice. i always have the trailer brake on (applied/checked) when i am hooking up or dropping. its the first thing i do when dropping and the last thing i do when hooking up. with the brake on you know the trailer isnt going anywhere so you are safe. personally i dont think its too much hassle for the peace of mind 
Use it, or lose it?
(Yes I know the unit is away from the trailer, this is in the process of recovery, where the pin had been pulled and the trailer pulled back away from the unit.)
nasty 