This is a great tool that most newer drivers don’t really understand and how to use it to make things easier for themselves when connecting and disconnecting from a trailer.
In most cases, you will have a switch to dump all air in the airbags which will lower the suspension of the truck a few inches.
When connecting to a trailer some times the trailer might be a little lower than normal and the 5th wheel might not be able to slip under the trailer.
When backing up and if you suddenly stop with a little bang, You can not get under the trailer till the trailer is lower than the 5th wheel. You can try to raise the trailer by cranking the landing gear or you can do the easy thing and hit the air suspension switch lowering the 5th wheel height. After around 5 seconds you can back up and when the 5th wheel is under the trailer, you can hit the suspension switch again and continue to back up till you hear the 5th wheel latches around the kingpin.
When you are disconnecting from a trailer you can use this to disconnect smoothly if done right.
There is this leveling valve that controls how much air is put into the airbags. The reason for this is because the trailer needs to be a certain height. If you have a trailer that is empty there will be a lot less weight on the air suspension than if the trailer was empty. The heavier the trailer the more air needs to be in the airbags to keep the trailer level.
When you are lowering the landing gear and when it touches the ground the weight of the trailer is still on the 5th wheel and your goal is to have all the weight on the landing gear before disconnecting from the trailer. You can either crank the landing gear down till most of the weight is on the landing gear or use dump the air suspension.
So when you disconnect from a trailer, in a split second there is no weight on the 5th wheel now but there is enough air in the airbags still in the 5th wheel for a heavy trailer and it does not adjust instantly so the 5th wheel is very high now and it will slowly adjust. So it is best to dump all the air from the trailer before you get form under the trailer. Once you are out from under the trailer you can hit the switch again, to put air back in to keep the air suspension level.