is anyone knows why my rx7 coolant will run back to my coolant reservoir after driving?

 

Air is getting into your system.

 

its designed to syphon back into the reservior when you shut the engine off and the engine starts to cool. If it's syphoning back too fast you may have other issues like too much pressure building in the cooling system, ie. faulty radiator cap or coolant o'rings leaking causing exhaust to leak into the cooling systen in turn over pressurizing the cooling system. In a lot of cases when the cooling system is over pressurizing it will over fill the reservior to where it will leak onto the ground.

 

Replace your rad cap with a new one.
If it gets weak or the rubber gets bad, it will allow the coolanet into the overflow at a much lower pressure.
It's a cheap easy starting point.