I had a similar problem. Coolant would dissapear. Cooling system held 15 psi overnight.

Had a hairline crack in a cylinder wall, when hit it would seep into the cylinder and go out the pipe as steam. Finally found it when the leak filled the chamber and hydrolocked on the next start up.

Odd thing was that somehow the water was pushed into the intake from hole 3, and hydrolocked #6. Found the crack on engine tear down.