Hmmm. That sucks. You are losing much fluid if it's like you say so I'm wondering why it's so hard to find out where it's coming from.
When you remove your radiator cap, can you see the coolant flowing strong from the passenger side of the vehicle to the driver side in the radiator? Have you checked around your water pump. If it is good, you should see good flow of coolant in your rad. Check for leaks on bottom and side of water pump.
Also, check your hoses. If it takes 6 or 7 days to leak, there could very possibly be a small leak in a hose in which the coolant escapes only when pressure builds up greatly. If this happens, first of all, it'll be difficult to see the green coolant b/c it will be steam and will not hang around and drip/collect like liquid coolant that leaks. Secondly, the hole could be so small that you cannot see it from just a quick look.
Try driving the car for a few minutes and get it good and hot. Park it and leave it running. Check the front and back on top and bottom of radiator and carefully inspect radiator hoses for minute steam leaks. Stick your head under the car and look around the bottom of the water pump. Check and see if there is any green chunky build up around the reservoir cap and radiator cap.
Hope you kill the problem.
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351W-powered 1979 Ford Mustang notchback
Stock 5.8L under 4" cowl, C4 w/ shift kit
Holley 750 cfm, Edelbrock Performer RPM intake
1 5/8" MAC shorty headers, Al driveshaft
2.5" Off road H-pipe, 2-chamber Flowmasters
Front: 225/60/15, Rear: 255/60/15 Eagle GT II
Weld wheels (15x6;15x8), 8.8" Rear w/ 3.55s
14 x 4” K&N air filter (getting the Xtreme setup someday)
"Red, thou art my companion. Hasten now your quickened metamorphosis to Green that I may conquer all who dare abide there beside me. May they be left thither behind burnt black." ---Fox Body