I replaced the water pump in my truck a couple weeks ago. What is that tank near the rear of the engine on the passenger side? Is there suppose to be coolant in there? It's empty.... I think there was coolant in there before.

 

Yes that is the coolant overflow tank. It is marked with the full hot level on it. I would fill it half way with the 50/50 mix and run it until it is up to normal temp then check the level and adjust.

 

When you open the cooling system, it's possible that you'll leave air in the system. As it pressurizes, it will draw coolant from the overflow tank. Do like rock said, keep an eye on it. The radiator will draw from that tank if the coolant level is low.