How about trying a pressure bleeder.  You can buy one from Wal-mart for around
$30.  Then you bleed from the bottom up and you can look for leaks while you
bleed and not have to pump the brakes and then try to hurry and look under the
van.  You could have a hole in the line somewhere, or in your caliper or in
your rear wheel cyliners.
Hope I helped.
Ken Wilford
John 3:16