So i replaced the water pump, radiator, upper and lower hose and the thermostat on my truck. My truck was running great but now i am seeing the the gauge in the instrument cluster is dropping below the half way point and even as close to the c mark while driving on the freeway. As soon as i get off the freeway it comes back up to mid way. I am thinking that maybe the head gasket is blown, i keep adding radiator fluid in the radiator and there is a lot sludge in the radiator. There is no white smoke coming out of the exhaust and no water mixing with the oil. i really don't know what else to do, i was thinking a full flush of the entire system, and see what happens. Any help will greatly appreciated

 

did you use plenty of RTV sealant on the water pump retension bolts , they go through the block into the cavity

sludge in the bottom radiator tank tells me that zero coolant is flowing through the radiator .

the coolant pressure is too much and

the head gasket is allowing coolant to be burned in the cylinders then passed through the exhaust

 

After warm up temp gauge should stay just below 1/2, if it goes lower at highway speeds then new thermostat is either broken or not installed correctly.
Use 192 or 195degF thermostat, NOT 180


Loss of coolant is plain and simple a leak, you just haven't found it yet.

First, never add coolant to radiator, you add it to the overflow tank, fill to COLD line when engine is cold.

If you want to check the rad that is fine, if it isn't at the top then replace rad cap, one of it's 2 valves is bad.
And check overflow hose, pull it off rad and blow thru it, make sure it isn't clogged, and make sure overflow tank is clean from debris that may settle at the bottom and prevent coolant from flowing back into rad as engine cools off.

When engine warms up coolant expands and rad cap lets it flow out and into overflow tank, when engine cools off rad cap allows coolant to flow back into rad, so even if there was a leak rad would still be full, overflow tank would be empty.