I believe the cooling system is designed to run at about 15psi with the valve on the coolant tank itself. If you pump the entire system up to 25 psi you could possibly cause a leak anywhere in the system, heater cores, radiator,hoses, etc. 

Secondly, if you have a tiny micro leak at the head gasket itself it may take combustion pressures wich are over 1000 psi to allow gases to pass by.  

25 psi from the coolant side or even charging the system with air compressor 125-150 psi and doing a leak down will tell you nothing if the leak is as the first stages where it will put air in the system while driving but not show up in tests. 

Retorque the heads, check the coolant for hydrocarbons, bleed it properly, say a few prayers, if its still getting air, 99% of the time you are going to have to pull the head and change the gaskets out. 

Doug

  ----- Original Message ----- 
  From: Raceingcajun 
  To: vanagon@GERRY.VANAGON.COM 
  Sent: Sunday, April 02, 2006 12:36 PM
  Subject: Re: Air Getting into Cooling system

      A quick way to test for head gasket leakage, is. Park car for the night,
  when engine is "cold", install a "Cooling System Pressure Tester" on the
  filler neck of the coolant recovery tank. Pressure up the system to about 20
  to 25 lb's, don't go any higher. Pull the spark plugs, and let it sit over
  night with pressure on the system.
      In the mourning, have someone turn the engine over while you watch the
  spark plug holes. If water/coolant "Spits" out of the plug holes, the head
  gasket on that side is leaking!
      If there is no water/coolant at the plug holes. Check the rest of the
  cooling system for leaks.
      A tester can be rented at most rental places, or maybe borrowed at a
  "FLAPS".

  Howard

  Subject: Re: Air Getting into Cooling system