Use a few squirts of penetrating fluid around the hex overnight is best.
Remember to turn the heater control to hottest position.
Keep straight downward pressure on the bit with the left hand, while using the long handle right hand, to break the seal of the hex screw.
If it doesn't break free, & you think you are going to strip the hex screw, stop there.
You don't have to open the hex bleed very far before coolant will start to come out.
It may take a few seconds for coolant to start to come out, so go slowly opening it & be prepared to close it.
What you are looking for is a
very small but steady stream of of coolant coming out, without air bubbles.
It generally happens pretty quick.
After cooling down all night, re-fill with Hoat to the cold line if necessary, & hopefully that stops the gurgling.
Good Luck.