BTDT.

Pressurize the system to 15psi (no more), and look for leaks). My sister in
law's vanagon had 2 leaks that sucked air, but did not leak coolant! Several
list members said this was not possible, but it was the case on this van!
Very very frustrating.

Specifically I found the rear heater core valve (bad; I replaced the valve
with a new one), and the rubber coolant hose connection to the plastic hoses
running forwards (I added a 2nd clamp).

John
Aircooled.Net Inc.