Can I use rubber transmission hose to replace a bad upper expansion tank hose (I have a piece lying around)? I don't know if it would withstand the pressure and temperature? Any thoughts?

 

It should work ok for you. Transmission hose is similar in ability to handle the temperature as I'm pretty sure transmission fluid gets hotter in normal use than coolant will and the pressure rating of the trans hose should be well above anything you'll see in the system as the radiator cap will limit it.

 

Which hose? The one out of the bottom or top thin one?

 

top thin one