First of all, I have to say that this question pertains to my '89 Lincoln, not my truck...but at least it's a Ford.
For some reason, I keep losing a large portion of my antifreeze (1+ gallons) out my overflow tank - even if I start with it completely empty, after I've run it 5-10 miles the tank is full to the top and I've lost perhaps a gallon or so out onto the ground. The car isn't overheating (according to the gauge, and also checking oil temps and such), and the cap is good...in fact, I've tried 3 different radiator caps and they all did the same thing. What would make an engine running at normal temps decide to start pushing all the antifreeze out into the overflow tank? The thermostat also seems to be working okay...at least, it's working the same as it was before I started having this problem (according to the gauge). That's what I don't understand...the car seems to run the same as it always has, but now it pushes all the antifreeze out into the overflow tank and then onto the ground. Once it loses all it's antifreeze, then the engine overheats (obviously)...but it loses all the antifreeze first. Basically, the engine isn't overheating and causing the antifreeze to boil out - the antifreeze all comes out first while the engine is at normal temp, and then the loss of antifreeze causes it to overheat later.
My gut feeling is that I have a cracked head (or block) or lost a head gasket, and air is getting pushed into the water jacket...which is pressurizing the cooling system and pushing all the antifreeze out. This wouldn't surprise me, as this engine has had substantial problems for some time now...and was severely overheated. However, I still have some hope that it's something less severe than that.
But, first get a reputable shop to 'sniff' the radiator/coolant for CO2 and other gases in the coolant. Generally it will be detectable if it is blowing all the coolant into the overflow... You might even see the bubbles coming out of the top tubes in a crossflow.
Same happening to me couple of days ago after a A/C compressor installing. When I run truck without compresor never made this, but after new compressor installing made failure. I put back some water (emergency) as antifreeze and will take to Shop for inspecting complete freezing system.
I´ve been running again (with A/C) couple times and is working fine, really fine. I assume there´s some dirty blocking a valve or something, Or the A/C freezer was not properly filled.
This may or may not work for you but I had the same problem and it was the radiator cap. The spring in the cap had lost some of it's spring and it was flexing and overflowing the reservior. I had never heard of it before but someone older and wiser suggested it and sure enough it worked. Got a new cap and all was solved.
I have a 2002 Ford Explorer and I am still trying to figure out why the coolant will overflow in the radiator reserve tank. I put in a new thermostat and a new cap to the overflow tank. When the tank is cold, sat over night, I put in new anti-freeze to the cold coolant line. Then I start up the explorer and let it run for a while. The temp gauge goes up, then drops a tad and cold air comes out of the vents. After I rev up the engine, I see the coolant level starts to rise in the overflow tank, then it goes to the top and starts to leak out. This takes a while being the thermostat has not kicked in to help suck the fluid in. Before this happens, the heater core lines are luke warm. After the tank overflows, the remaining gets sucked into the engine and I get nice warm heat. I have no idea what causes this. I had the test done to see if the head gasket is bad. The test was negative. Anyone have any ideas?
If the reservoir goes low after it 'normalizes', have you put coolant into the reservoir so it is there as the engine cools? If not, the engine/radiator will draw air into the system and it can displace coolant added after cooling before you start out next and the engine warms.