I did NOT do a flush. I used the following burp process below. The car has driven perfectly with it!
Thanks to c6zhombre for this burping process "i got a reische unit a few days ago and decided to install it today. i also wanted to at least drop what i believe is factory fill 5.5 year old coolant in my '04. so i ended up doing the "flush" 3 times with 2 gallons distilled water and then finally a dist water/coolant mixture the last time i filled it.
one thing i noticed during the burping was i just burped it during the initial fill with the motor off. i would fill exclusively from the crosstube with the degas cap off. when the water level got to the fill line in the reservoir, i would put the degas cap on. then keep filling the crosstube until water hit the top threads. then i would reach and unscrew the degas cap, and "gulp", it would suck down at the crossover tube and i would retighten the degas cap. i kept doing this, usually about 5-6 times until unscrewing the degas cap had no effect, then i would plug the crossover. It would consume about 3/4 quart during this burping (which takes maybe 3-4 minutes total to complete the entire burping process). i also had the car front on ramps. i never had to burp with the motor on, the car drove perfect each of the 4 times i flushed today (never got over 182 temp, cruised 65mph at 174)."
