I’m gonna throw a spanner in the works.
What is the shunt button for and what effect does it have on the trailer.
My guess would be that it dumps the trailer air to release the brakes so it can be shunted around without needing to connect the sussies.
Would I be a million miles out?
Dean
Assuming there’s enough air in the tank to fill the brake chamber then as you say it enables you to release the trailer brake without having to connect the air lines.
dar1976:
I’m gonna throw a spanner in the works.What is the shunt button for and what effect does it have on the trailer.
My guess would be that it dumps the trailer air to release the brakes so it can be shunted around without needing to connect the sussies.
Would I be a million miles out?
Dean
Half right. It doesn’t dump the air, because trailers have spring brakes, and dumping all of the air puts the brakes fully on. It uses stored pressure to take the brakes off. However, there has to be enough stored air in the trailer to be able to do this.
Thanks both for clearing that up.
What is the difference between pushing the shunt button in to release the brakes, and pushing the brake button in to release the brakes? I’ve never really got my head around why a shunt button is needed. Presumably, if the air is exhausted in the trailer tanks, neither will release the brakes anyway? 
Gary
scaniason:
What is the difference between pushing the shunt button in to release the brakes, and pushing the brake button in to release the brakes?
You can get the trailer to move without having the red airline connected with the shunt button.
scaniason:
I’ve never really got my head around why a shunt button is needed. Presumably, if the air is exhausted in the trailer tanks, neither will release the brakes anyway?
That’s correct.
Harry Monk:
scaniason:
What is the difference between pushing the shunt button in to release the brakes, and pushing the brake button in to release the brakes?You can get the trailer to move without having the red airline connected with the shunt button.
Ah thanks, it makes sense now!