[oldstimer]

For what its worth, this sounds familiar to a well known problem with 97-99 Chevy small blocks. There would be a puddle of deathcool every morning. While the engine ran, no leaks. Pressure test fine. Next morning, a puddle of deathcool. It turned out the intake design was changed in 97 and it contracted at a different rate than the rest of the engine when cooling off. For the first few years of use, the elasticity of the intake gasket would make up for any difference. After the intake gasket became brittle, deathcool would leak, but only while the engine was cooling down.