Volvo breaks don't work when applied, then slams to a stop.

Are you talking about the cruise not disabling when you hit the brakes, or are you talking about what sounds like ice in the air brake system?

 

Are you putting pressure on the pedal, nothing.
Little bit more pressure on the pedal, nothing.
A little more pressure then BAM, it jerks to a stop?

Where have you been running, what kind of temperatures, and how often do you bleed your air tanks?

It sounds like you need to park the truck in a heated shop for a few hours, then bleed the tanks and pump the brakes a bit. I'm guessing you've got ice in the system.

This can be dangerous.

 

Assuming it's an automatic, get it rolling about ten mph in a dry parking lot (not snow or ice) and pop it in neutral and try to gently stop it and if it stops smoothly, it's probably some computer glitch that will require some nerd with a laptop to figure out. But I don't get how the cruise could have anything to do with it because you don't have the cruise on at 4 mph at the scale house.

If it does that jerky #### while it's in neutral, then it's the brakes.