Normally there is only one valve. Sounds like you didn't drain the block and heater at the same time.
Here's how I've done it in the past.
1. Drain the radiator.
2. Remove the hoses leading to the heater.
3. Flush the heater under light pressure just to make sure you don't damage it. Sometimes they will leak if they have any kind of blockage.
4. Remove and replace the t-stat (I do this whenever the coolant is changed)
5. Flush the radiator until clear water is coming out.
6. Connect all hoses and close the valve but keep the fresh water hose going in the top of the radiator. Make sure the radiator is full. Turn the heater lever to full hot, no need to turn on the blower more than just low. We just want the water to flow through.
7. Start the truck and let it idle keeping an eye on the radiator. You'll be able to see when the t-stat opens up, there will be a sudden drop in volume in the radiator.
8. Feel both heater hoses, they should be the same temp (hot).
9. Check your temp gauge in the truck. Should be normal range.
10. Turn off the truck and drain the radiator.
11. Normal mix is 50/50 anti-freeze and distilled water. However, you'll have a lot of water still in the motor and heater. So make the mix a heavier on the anti-freeze side. Or you can pull the hose off the lowest point on the motor and let it drain out.
Not really hard to do, just a PITA sometimes.
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