There is an expansion tank that needs to have the right level in it.
![](./images/smilies/spacer.gif)
Then top up via the coolant neck and bleed from the dad and the heater valve in the trunk to get all the air out.
![](./images/smilies/spacer.gif)
Heater needs to be on hot to do this.
![](./images/smilies/spacer.gif)
Once all bled and levels correct the coolant will expand and if the pressure exceeds the cap some will flow into the expansion tank.
![](./images/smilies/spacer.gif)
On cooling the system will draw back coolant from the expansion tank.
![](./images/smilies/spacer.gif)
There are plenty of guides to bleeding the mr2 coolant.
![](./images/smilies/spacer.gif)
If the engine is pushing excess
![](./images/smilies/spacer.gif)
coolant out it may be tired cap or head gasket issue pressurising system.
![](./images/smilies/spacer.gif)