You're worrying about the cooling system on a T25. It takes 16 litres, I think. The radiator is ancient and the pipes are very long. Release the pipes that come into the engine bay and stick a hose on there and flush for some time.
It may be that the rad is knackered - how old is the van, because that's probably the age of the rad.
Flush it, flush the block and see how you go.
You have jacked up the front of the van and released the bleed screw on the radiator, to allow the system to bleed, once you've filled it up?
Unless you do that you're unlikely to get it properly filled and run up.