The trailer is slow building air lately.

Typically caused by restriction in the lines preventing air flow to the valve. Trailers that have sat around with the gladhands open are prone to getting dirt, leaves, bugs, etc. in the line. Depending on your location it could be frost also. If you're far enough down south and sure it's not frost take the emergency line loose from the gladhand and from the valve itself and put air to it, both ways if possible. Check it real well, otherwise you'll force that debris into the valve and then you have to take it apart to clean it out

 

If you never change trailers it could be on your truck side as well. I've had water build up in the air line ahead of where they fasten to the frame behind the cab. Rare but have seen it. If you get your lines all loose at each connection get a jug of air line antifreeze with a little funnel and pour down everything before blowing it out. Leave your gladhand disconnected with the line down far as you can and hold the trailer button in on the dash for a good while. Then put a little in the line and send air to that valve. May help to loosen it back up internally if nothing else.

 

Hook your tractor to another trailer and see if it does the same thing. I fought a similar problem for a long time and it had nothing to do with the tractor, it was the main valve on the trailer at the air tank. They are cheap (cost wise) and prone to failure.

 

park valve on trailer is junk or contaminated most likely