Hey guys, I appreciate any advice here,

My coolant was brown in my 2011 Ford Explorer XLT, 3.5l v6 and obviously bad (no oil but definitely too old), so I changed it today, I made sure to flush it properly over and over again until it was coming out clear using a hose. Eventually when it was finally coming out clear I let all the water drain out (or so I thought) until it was just a drip coming out of the radiator drain. I went to refill it with new coolant but so far I have been only able to get my car to take 6 quarts of coolant (50/50). It’s supposed to take 12. I have spent hours trying everything to be able to add more, revving the engine, driving it around, squeezing hoses to purge air, everything I could think of and it has only hoped me to be able to add maybe 1/2 a quart. I am starting to thinking that somehow I didn’t get all the water out and there is still some left. Will that damage the engine? Do I drain the coolant and make sure all the water gets out this time? Do I just drive it and add as it goes down? Or do you think there is something else I need to do to purge the system of air or something? It continues to go down but like extremely slowly, like maybe 1/8 a quart per hour if that I am able to add. Please help, any advice is appreciated.