1980 300SD Cooling System No Pressure but overflow?

[Assault]

Hello,

This has baffled me since the day I bought the car. Occasionally coolant will exit the system via the overflow hose. Now the coolant amount can be at any level near empty or filled to capacity. It will exist the system via the overflow hose. Now it gets weirder. There is no pressure in the cooling system. I have removed the cap directly after running the car and shutting it off and there was zero pressure. Not even a slight his. Where should I proceed? The temp gauge is indicating that the thermostat is stuck open (although I have hot heat). I have verified it is exiting the expansion tank via the overflow hose because I have observed the hose while it was leaking out of the system and saw watched the hose wiggle and slightly bulge as it passed the coolant through.

I have replaced the cap with a brand new cap(worked flawlessly on m other 1980 300SD, now a parts car)
I have also replaced the hose that connects to the top of the radiator and goes back into the expansion tank.

Thank you!