I read krankies excellent article on coupling up.
Can anyone explain what the shunt valve does 
To understand the how a Shunt valve works. You first have to understand how a Spring Brake works.
A Spring Brake is effectively an actuator with two chambers separated by a diaphram which contains a massive coil spring. In the absence of any air pressure, the spring will APPLY the brakes. The force required to release the brakes is typically 6 bar. (6 x atmospheric pressure). To then apply the brakes requires a pressure on the opposite side of the diaphram that is greater than the pressure that is required to release the brakes. i.e. >6 bar. (Typically 8-10 bar).
Because every trailer has it’s own air reservoir, fed by the Red line, there should be available an air supply greater than 6 bar. Pushing in the Shunt valve will utilise this reserve of air to release the trailer brakes.
However. There is a parallel thread on the PDF about ‘split’ coupling. One way to avoid that is to couple up to the trailer in the normal way, push in the shunt valve, pull forward turning sharply, and then connect the suzies. However, this presupposes that there is sufficient air in the ‘trailer’ tank to release the brakes in the first place.
Different trailers supplied by different manufacturers have different characteristics, and I have previously disconnected all of the lines only to find that the trailer in question WOULD NOT move. Even on a brand new trailer.
Some trailers, even when connecting the Red line, will not release the brakes unless the electrical suzy is connected. (24R. 24U, Someone will correct me. - I must look up the pinouts.)
There is no hard and fast rule. You just HAVE to get to know the fleet that you are working with.
Krankee.
Many thanks for that explanation. Unfortunately you lost me in the first paragraph.
My understanding is that if you disconnect the red air line the air drains out of the trailer tanks and the brakes lock on (is that correct?).
Am I correct in thinking if the shunt valve is pushed in (or pulled out) you can move the trailer without the red line being connected.
Sorry if I seem thick.
Cheers Ken.
To put it in ley mans terms
The shunt valve temporerely releases the trailer brakes when un coupled
Hows that m8
Yes, pushing in the shunt valve will release the trailer brakes. 
Krankees answer was a bit long winded 
and I think , didn’t give you a straight answer. (Sorry Krankee)
Try this for a bit more detail:-
A brake chamber is a sealed unit, with a diaphram splitting it in 2 with the brake operating rod attached to the diaphram. There is a powerful spring in there as well. (There’s a bit more to it than that though.)
When you uncouple your red line, that releases the air pressure on one side of the diaphram, the heavy spring pushes the diapham (and so brake rod) into the space and puts the brakes on.
When you push in the shunt button, the air in the tanks is allowed to pressurise that side of the brake chamber again, so your brakes release.
When driving down the road, when you put your foot on the brakes, higher pressure air is sent into the other hakf of the brake chamber, so your brakes come on. This side has a pressure regulator, so the more pressure you put on the pedal, the higher the pressure allowed into the chamber, so the more braking effort.
Simple 
really.
Why all the technical stuff Simon (dont want to tramp on your feet)
Was i right the first time i explained it or not
4get the big wig talk all it does is release the trailer brake
Am i correct or not
Sorry m8 hes a new trucker he needs to know
Yes your answer is right, CPCMan.
But a bit of basic tech can help in another, different situation, if you use your loaf, I think.
Many Thanks for all your replies.
I’m still not exactly sure how the valve works. I’m A bit thick when it comes to mechanics 
. But now I know what it does. Which was really all I wanted to know.
Thanks Again.
Ken.
ah ha the shunt brake.
real handy when your doing a trailer swap, saves you putting the air lines on,
nearly hand my fingers burnt once in relation to the shunt button, i didnt check to see if the trailer park brake was on before coupling (muppet i know), so i started to back under the trailer and guess what? the shunt button had been pushed in, so low and behold i had a runner on my hands, just snatched the pin in time,
can only think that someone hand mistaken the shunt button for the park brake…lesson learnt allways have and allways will check that the park brake is on before coupling from that day on…phew close call
regards
manny
Shunt valves can be used to move trailers without airlines attached to allow space for all the lines to be coupled with the cab turned, allowing the driver to get into the small gap between the cab and trailer as there are very few day cabs around.
Mavis mods to the trailer make the coupling much more safe, the couplings can then be slid to one side of the trailer, the driver can couple with their feet on the ground, last year we had one fatal accident and 27 minor accidents involving coupling trailers at the national firm I work for, minor accidents involving coupling can include slips trips and falls from the coupling area, it’s a dangerous task!! especialy if your concentration is broken.