Coolant vacuum
 

Just finished installing an engine and trying to fill with coolant. When put under vacuum, it pulls in ok, but only gets to 10 pounds, then it seems something clicks and drops back down. There are no leaks.
Is this a problem with the relief valve?
Thanks.

AutoMatic or Stick?

Manual. And fresh AOS too (from Porsche.)

I had a similar problem recently after changing the coolant tank. Turns out, the Bleed valve was defective.

Do a pressure test. If it doesn't hold pressure, clamp off the overflow tube and fill the black top of the coolant tank with soapy water. Mine bubbled around the bleedvalve and when I took the valve out, was cracked inside. Replaced and held pressure. AIrlift vacuum held at 15 mm Hg.