Poor braking

I think there may have been a relay or doodads installed into your braking system to limit the loss of air when empty. Theoratically you have just lost your trailer and is now bleeding through your red glad hand. The engineers may have built your air distribution system with a relay or few to prevent you from losing too much air too fast.

My theory is if you were to remove this protection you might find the whole of the primary air supply now available to your braking. Just like it was old school.

 

Maybe if you were able to put a dead glad hand into the trailer ones so that your truck thinks it has a trailer attached to it. It's brutal to hack a system this way. But it's from my little bag of tricks I don't reach into often when I am with a tractor that wont stop.

I did write my first post poorly but I think from your answer you are on the track I think you need to be on. Consider it carefully. Who knows it might work out well.

 

No clue as to what the problem might be, but that is a sweet looking truck.

 

You could have an issue with the R-14 relay valve, this would cause issues like hard braking.

The R-14 relay valve is separate from the trailer relay valves, so if there is an issue, it could be with the R-14.

 

Disk or drum brakes?