The brakes are sticking on, Steve either due to the caliper pistons sticking, or the caliper sliding pins likewise - however you say that you have replaced the caliper. Was it a new one? If it was, after driving the van, jack it up on that side and see if the wheel spins freely. If not, undo the bleed nipple a fraction, let a bit of fluid out and see if that works. If it does, then either you have a swollen and blocked flexible pipe or a master cylinder fault.
John, July 2009