I would first disconnect the lines to the master cylinder and flush it out separately from the rest of the system. You don’t want to introduce any PS fluid downstream if you can help it. Heck, I would probably just replace the master cylinder. Rebuilt units are pretty cheap. Then flush the entire system.
However, if you use the van (and, presumably, the brakes) already, you may have contaminated the whole system. Elementary, really. ;-}