Can't refill coolant after flush

I drained the radiator. The expected amount came out. Then poured a flush agent in the recovery tank, filled in some water (I was using a hose and didn't keep track of how much went in). After running the truck and redraining, I can't put the correct amount of coolant back in! I only got about a gallon and a half back in. The recovery tank is full (overfull) but no more will go back in. This truck calls for 4.5 gallons! The radiator is sealed, so the only way to fill it is through the recovery tank. I live in Florida, it is hot here. I have driven it for 2 days and the truck runs at normal temp, both front and back heaters blow hot air. What gives?? Please any help!

your system may hold 4.5 gal but unless you are draining block your about right on amount it should hold 1.5-2 gal.if all is fine with level & no air in system your ok & would'nt worry

My worry is the truck has 150K miles on it. It has never been flushed. I didn't back flush the block and heater core. I used to buy that kit that attaches to a garden hose to flush it. Didn't want to do that this time. I really want to get all the old stuff out and fill with all new fresh coolant. How can I get it all out?

To get a good flush and all of the flush stuff out you need the flush kit or run it with a heater hose off and keep flushing it untill it is clear.

It is hard to drain all coolant or water out.
Remove lower rad hose to help aid in draining.

That should let you get enough straight coolant back in.
Do not add the 50/50 mix stuff.
If a 4.5 gal system try to get 2 1\4 gal or more of staright coolant in it.

I drained the radiator. The expected amount came out. Then poured a flush agent in the recovery tank, filled in some water (I was using a hose and didn't keep track of how much went in). After running the truck and redraining, I can't put the correct amount of coolant back in! I only got about a gallon and a half back in. The recovery tank is full (overfull) but no more will go back in. This truck calls for 4.5 gallons! The radiator is sealed, so the only way to fill it is through the recovery tank. I live in Florida, it is hot here. I have driven it for 2 days and the truck runs at normal temp, both front and back heaters blow hot air. What gives?? Please any help!

if this is a 1999 on up vortec v8 there are 2 drain plugs at the back of engine where the block heater would be mounted, with an allen hex head wrench 17mm you can drain block completely....the drain plugs are about 1.75 inches in dia...