My lt1 will overheat quick. I replaced water pump thermostat but there is no pressure in the system so no coolant is getting into the block. I have tto Rev it up in order for the thermostat to allow coolant into the block. Any suggestions why it is like that

 

Replace your radiator cap to pressurize the system. There was a guy on here recently who forgot the waterpump coupler when he installed his new waterpump, did you do that too?

 

If there's no pressure in the system that means you have zero coolant in the system which is highly doubtful. It reads like you either have a leak where coolant is escaping. You need to fill the system properly. Do a search here. Shoebox may have something on his page as well- www.shbox.com
You then need to take the plugs out and do a pressure test. You can rent a tester at any parts store. Do a search on pressure test as well.

 

We recently replaced the radiator cap. which should be 18 right? but the radiator cap feels kinda lose and fairly easy to pop off

Also when we put in the thermostat we dumped over a gallon of coolant into the water pump without tthe thermostat so it would go into the block and it could still of taken more we then put more coolant into the radiator could it still need more coolant I know the say. The block and radiator together take around 15 quarts