Both sides behind slightly under pressure. Exhaust manifold, I guess that could be. But I would think you may have become aware of that as it could have been leaking into the cylinders?

Are you sure you aren't losing coolant through the expansion tank, either on the engine or a remote expansion tank? Either one might bleed off a few ounces of coolant through the pressure caps and down an overflow hose?

Just thinking out loud, Eddy.