2013 W900L very poor bobtail braking

You can check by putting a quarter between your gladhands as you connect them together and then push in the red button and see if you get better braking.
If it makes a difference you can mount blocked gladhands on the frame/back wall to be used during bobtailing.

 

Never heard of the quarter deal. That is easy.
As to the problem the piston under the foot valve can cause some of those feelings. No brakes, no brakes, oh crap.

 

When your brakes finally grab when standing on the pedal does it feel almost like hitting a wall? Say when approaching a traffic light.

I had that problem and called Bendix, they said to replace bobtail proportioning valve with an (pretty sure but, will look in my maintenance records to confirm) R12 valve.

There will be a small air line left extra after the conversion, they said no need to plug it. After this my tractor stops the same as when the trailer is on. tractor is also Paccar, 97 379.

Again, I will double check this information later and be able to give more detail. I went through this around 08, 09 quite a few years ago.