There is no pump inside the overflow tank. As the engine can use more coolant, vacuum will suck up coolant from the overflow.

You may want to invest in a 180 degree thermostat and flush the coolant. Purple Ice or Red Line water wetter will also drop the temps.

If you just bought it there is no telling how long its been since the radiator was completely flushed. Do that first along with the Redline water wetter like ghostdog suggested.
If you still have problems you can get a high flow water pump like the one from Edelbrock. Its cheap and it cured my overheating problems.
Oh -- and that coolant overflow tank has a fluid level sensor inside, which might be what you thought was a pump.

Im sorry i forgot to mention that i had the system flushed and installed a 160degree thermostat from autozone.
I reinstalled the flex fan properly.

But my question is why didnt the fluid return to the radiator from the overflow bottle?

Couple possibilities...
1) You have a crack or leak in the over tank and or in hte line which returns to the radiator
2) You have a leak somewhere else (ie the heater core or water pump) and the coolant is returning to the radiator, but leaking out in other areas.

Good Luck

i was told that a bad radiator cap will prevent the fluid from re-entering the radiator from the overflow bottle thru the little hose in the neck.
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i'll buy one tomorrow and try it.

ok..i changed the radiator cap, and the stang runs a little cooler, but when I "get on it" on these hot days the temperature rises considerably, the same with the air conditioner running. Could this be a waterpump problem or radiator problem?

Worn out fan clutch!!!!

I just need to clarify that you understand the purpose of the radiator bottle. First, it always has coolant in it. If the level drops below cold, and you filled it to the cold level when it was cold, then you should worry. If the level stays the same all the time, don't worry about the overflow. Next, if the guage says it's running hot, you need to verify it with something like an infrared temp gun. Do that, then let us know the results.