It could well be the slave cylinder and on most vans after 1999 these are inside the clutch housing so the gearbox will have to come off.
Do you have any clutch fluid leaking from the bottom of the clutch housing at the joint where it bolts to the engine. (there is a small drain hole in the casing) as that's a sure sign of a failed slave cylinder.
Other than that it could just be that you haven't bled all the air out of the circuit. they are known to be awkward with this.
Phil.H
February 2010