On my 87 Toyota truck the coolant goes into the overflow tank and I have to add

[T.F W]

coolant ,why doesn't it return? this causes the thermostat to open later.

 

[I'd rather be outside]

The coolant reservoir has an overflow tube that drains under the truck. Have you seen any small puddles of coolant under the truck, usually after you shut off the engine after its well warmed-up? If so, the coolant system is just bleeding off the excess coolant.