Fill the system, as full as you can, with coolant, and distilled water mix, 50/50. Attach a clear hose to the valve in the radiator. Find the correct size wrench for this valve.
Raise the rear of the Van 18" (min), then, after starting the motor, open the heater valve fully. After the thermostat is fully open, you will see coolant, and air escaping from the front valve in the radiator.
With the heater valve open, and the blower motor on, when you begin to feel heat, the system is close to being blead, may take 20-30 mins. As soon as no more air is visible coming from the clear hose you installed in the bleeder valve (coolant only coming out), close the valve. Make sure the tanks in the motor compartment are full to the correct level. Main, with the pressure cap - full, aux. up to the correct level.