This seems simple and I'll admit I am probably overlooking the obvious.
Found aa leak in heater core, replaced it. System was fine for about 4 months. Coolant level is dropping. No steam at tailpipe, no water in oil, no hoses leaking, radiator is new as of 6months ago, can't detect any leaks in it. No coolant under vehicle. Pumps 16 lbs on system holds for over an hour. Cap was same way. It is a lever type cap when engine is at operating temp, pressure is on system when vent lever is raised. Could the system be pushing past the cap out the hose onto the ground while driving, takes about 2 quarts a week. Any ideas.

99 Lhs 3.5