I was about to change my antifreeze because it is almost 2 years old, and I had all the plastic off since I was changing gear oil and engine oil tranny. I’m a little confused why it says to clamp both hoses, I understand about cracking open the bleed screws, I ‘m not sure how to access the back bleed screw but that’s another question, but why clamp the hoses, or maybe for the fluid to bypass and fill from the bottom up to the thermostat. Anyone have an answer, also how often do you drain your coolant.