The radiator in my wife's 1998 Accord EX 4 cyl cracked and we replaced it last weekend. It had been losing coolant and some was added, and when we replaced it the overflow tank was nearly empty but there was still fluid at the neck in the radiator. We recovered around 3/4 of a gallon of coolant from the radiator drain valve and the little bit in the overflow tank. When it was refilled about 3/4 of a gallon went in. (Filled until it came out in a stream from the bleed valve near the thermostat.)

So, the coolant was quite old at that point, like 14 years old. pH was 8 and DC voltage wasn't into the "corroding" range, but still old. About a quarter of a gallon had been added when it was losing fluid. Seems like there is around 1.25 gallons that didn't get changed out, slightly diluted with that little bit of newer coolant. I don't want to mess around with the drain bolt on the motor. Would one more drain and refill of the radiator be enough to count as a full "coolant change"?

 

Hmm.

I've never been successful bleeding with the thermostat bleed..its too low.
​​​​​
My opinion, turn cabin heat to max, remove the thermostat, use a garden hose to blast water thru the whole motor and cabin heater core. (Motor off). If it's that old, you should do a quality flush.

The drain bolt on the block is no big deal. You can remove it at least 10 times without replacing the washer and not have any issue with leaking there. Don't be afraid to remove it.

Bleed the cooling system by keeping cabin heat to full, running motor with radiator cap removed. Continue to top off the system until the radiator fan turns on, then cap it up and call it a day.

Coolant is cheap and water is cheaper. If you were running old green consider going to new blue or even pink (best imo)