Someday you may want to change the antifreeze in the Universal M25 diesel, or like me, you HAVE to drain it because you are going to replace the exhaust riser.
BTW - what's that small hose coming out of the manifold under the coolant cap? That's the overflow hose for the manifold. On Reality Check, this hose is routed down the side of the engine and ends up in the bilge under the galley sink. There isn't a coolant recovery tank, never has been, and there has never been antifreeze anywhere it didn't belong (in the manifold).
Hey look! The petcock handle is facing forward, in line with the hole where the antifreeze comes out! Yes, that is the closed position.
I can open and close the petcock by pushing it with my thumb. Some people use pliers.
One cup isn't going to hold all the antifreeze in the manifold!
Fill up cup, close petcock, dump antifreeze in container for disposal, repeat.
What next? What about getting the antifreeze back into the manifold?
Taking off the heat exchanger is pretty common and you have to drain the coolant to do that... the next step after removing and re-installing the the heat exchanger is refilling the manifold with antifreeze and bleeding the cooling system.
Most marinas have a recycling area for disposal of old fuel and oil. Antifreeze, no. Check Earth911.com for recycling centers for antifreeze near you.