Slow Air Build Up With Heavier Loads

Good you found it, but I have to tell you the dealers are going down hill with mechanics. I been dealing with a freight liner dealer for years and had great service out of them but the last two trucks that went into the shop for repair never got really repaired, total cost was a lot more than $900. They just replaced parts until it stopped throwing codes, then said it was fixed. A day after it was returned, one of the truck broke down under load and towed to another dealer who found the problem and it wasn't something that they even considered.

 

One easy way to test a leveling valve for leaks and to see if it will shut off with trailer load is. Have air system fully charged and springs inflated then run trailer landing gear down to ground and with them in low gear crank down to lift up on truck suspension a little at a time. Valve should fill to compensate and then shut off when it goes back its “level” set position. If it stays open it is bad. Then when you crank gear back up and put trailer weight back on suspension the valve should open to deflate to level back out and then shut off. If it continues to leak and drop below level set position it is bad.
You can do this with engine shutdown in a quieter environment so you can easily hear whats its trying to do.