Assuming the draw bar is extended, push back and you should feel a very firm resistance, however slowly but surely the draw bar and therefore the damper will move back. Once in as far as it will go, it should then slowly bring itself back forward and this will prove the damper is working correctly.
If the vans 'banging' when you brake then it's either the damper or to much slack in the linkages, you can check for this slack by seeing what clearance there is at the back of the draw bar where it contacts the actuating arm, this gap should only be a few millimetres and not the inch I found on my new van!!