yup. get a pressure tester. test it hot AND cold.

I had the same problem in my 383 block.

got a tester, and the darn cooling system held 15 psi overnight. didn't lose a single pound of pressure after a good 15 hours.

a week later I bent a pushrod as I hydrolocked a cylinder that filled with water after a 5 minute drive.

the #3 cylinder wall was cracked, and when it heated up, water would push into the cylinder and go out the tail pipe as steam. but when cold, the metal contracted and 'sealed' the crack so that it held pressure. no water in the oil either.