apologies to the air-cooled crowd, i'm sure you're getting tired of 
all this.  and i bet you really want to trade-up for a wasserboxer!

Several tips have been previously posted on bleeding the system.  
here is one i think works.

the Washer Pump method.
1.  prep van per Bentley by raising front, opening bleeders, heater 
cores open, etc.
2.  drain the non-pressure overflow tank by sucking out the 
anti-freeze with the contraption decribed below.
3.  remove the overflow hose from the non-pressure reservoir tank.  
attach this hose to a 'replacement' washer fluid pump (about $12)-- 
the other end will be connected to the pressure cap (works from '84 on, the 
'83 1/2's had a different setup).
4.  attach a piece of hose to the suction end of the pump and place 
this in a jug of 70/30 anti-freeze/water mix or just straight 
anti-freeze.
5.  power up the pump (12v available at the alternator stud) and it 
will begin pushing anti-freeze into your system.  open the radiator 
bleeder screw and let the air out.  the pump will continue to push 
anti-freeze into your system and displace air.  do the same to the 
rear heater bleed screw.
6.  you may find it necessary to start the engine, warm it up, and 
have the coolant circulate a while to move the entrapped air to the front.  it 
is a tedious procedure.
7.  if air is in the pressure reservoir, just crack the cap open 
slightly and it will burp it out.  when you're done, hook the hose 
back up and refill the overflow tank.

this procedure makes for a much cleaner operation back at the engine 
and i think ensures a more positive bleeding of the air.

while you are fiddling around with all this, take time to ensure your 
cap is operating properly.  you should be able to blow air into the 
nipple sticking out of the cap, and it should hold pressure when you 
suck on it.  anti-freeze is toxic, remember, so be careful to wash it 
first.

-dan