Well, think of all the places coolant goes and think of what signs it might leave. Start from the radiator and progress through each part of the entire engine back to the radiator. Did you have the head magnefluxed while it was off? (You would have mentioned that.) Apparently you don't have any overheating going on until the coolant is nearly gone.
Radiator: signs of leakage top, bottom, left, right, front back.
Lower radiator hose: Seepage at either end or drops in the middle. Sometimes leaks don't appear until the engine is good and warm.
Water pump: Leakage on the front of the engine.
Heater tubes: Rust and moisture.
Heater coil: Moisture in the cab floor or on the windshield when using the defroster.
Oil cooler: Moisture behind the oil filter mounting area, water in the oil.
Engine cooling jacket: Leaks at the front or back of the block, moisture in the tail pipe or steam in the morning with a cold engine, water in the oil. Also, super-heating the coolant and boiling it out while you drive.
Head gasket: (already changed) Water in the oil, super-heating the coolant and boiling it out while you drive, or leaking it into the oil.
Head: It might be warped and changing the gasket will not seal well around cylinders 3 & 4. A crack in the right place might cause coolant to get sucked into the cylinders with the intake air.
Thermostat: moisture around the thermostat housing.
Upper radiator hose: Moisture at either end or the middle.
Radiator cap: (already changed) coolant reservoir overfilling.
I have probably missed a couple coolant paths. Good luck. Sometimes coolant leaks are tough to find when they are not completely obvious.
Last edited by Captain Dean; 02-22-2010 at 11:48 PM.