You've got an airlock, Steve. This is common when 119s are refilled. Air is either caught in the top of the radiator, the big hose at the top, or top of the block.

* make sure you refill through the balance tank and not the radiator lid
* fill, run the motor (short time), fill some more. It may take 3-4 cycles, just keep adding as the system soaks it up.
* Leave a little bit down, when it's flowing well, add the antifreeze. I suggest you use only Merc branded, my later model 140s use brown

It'll probably gurgle and bubble as the air gets flushed. Suggest you don't run the motor too long with no coolant (2-3 mins max)