I know all about the push/pull valve, and sometimes you have to plug in the emergency line to the other connector, and apply the trailer brake etc,
But do you move forward or backward?
Also, forgive this dumb question, how is it possible to move a 20’ box from the middle to the rear (for off-loading purposes), but it’s not possible for a crane to place a 20’ box at the back, and then the driver move that box to the middle?
You don’t move the box, you unlock the bogie and back up. You shorten the trailer.
To go from a 20’ to 40’ you pull forward - you need to lengthen the trailer.
You should never be driving with your trailer set at 20’ unload position, it’s a pain 
The problem is I had to take a 20’ box to the port, take that off then put on a 40’ box 1 min later
I had the twistlocks all ready for the 40’ but did not know you had to alter the trailer
Another problem I had today, I could movea 20’ box from (D) unladen all the way slide to the back, but I could not get it to go 6 inches from (D) to (C) laden. No idea what went wrong, gave it plenty of grunt (!)
Make sure the trailer and unit are level, as if the trailer is lower than the unit then you are trying to pull uphill and it can lift your drive axle and you’ll lose traction. I learnt the hard way years ago
And prey you haven’t a stiff trailer I had to chock one to extend it as I was dragging the locked wheels across the yard
Was at a place with a fellow driver and he said position (C) and (D) don’t really matter, 6" from each other
I had assumed there was some important design reason for using C (the positions for 20ft, 30ft, 40ft etc are all on a plaque) but he just stretched the trailer to D after loading and drove on
Maybe I’ve been stressing for nothing
If you are loaded with a 20ft and its heavy I’d have it set to to 20ft position just in case
Simple:
Get out and depending on the trailer you will have to remove the red line and put it on another point which automatically applies the brakes and removes the bolts holding the trailer together or you will have to do the apply the brake and push the other button.
For 40 and 45 boxes you need the trailer fully extended as far as it will go. A 45 and a 40 are the same twistlock settings, the only difference is the overhang when the box is on.
You’ll never use 30’s so don’t worry.
20’s go in the middle of the trailer. And if you need to slide the trailer back to load a 20 make sure the ‘arms’ for the 40ft setting that stick out the rear of the trailer are folded away or else the trailer won’t slide all the way back.
If you can’t get it to slide or can’t get it to lock just shunt/bump the trailer by trying to pull away with the brake on and it will move the trailer enough for the pins to move.
stagedriver:
Simple:Get out and depending on the trailer you will have to remove the red line and put it on another point which automatically applies the brakes and removes the bolts holding the trailer together or you will have to do the apply the brake and push the other button.
For 40 and 45 boxes you need the trailer fully extended as far as it will go. A 45 and a 40 are the same twistlock settings, the only difference is the overhang when the box is on.
You’ll never use 30’s so don’t worry.
20’s go in the middle of the trailer. And if you need to slide the trailer back to load a 20 make sure the ‘arms’ for the 40ft setting that stick out the rear of the trailer are folded away or else the trailer won’t slide all the way back.
If you can’t get it to slide or can’t get it to lock just shunt/bump the trailer by trying to pull away with the brake on and it will move the trailer enough for the pins to move.
some of our trailers the 45 setting is shorter than the 40 and on a 45 you use the very front twistlocks