It sounds like you are loosening the bleeder screws when the engine is hot. That is a potentially dangerous and inappropriate thing to do. The way to bleed the air out of the system is to remove the radiator cap when the motor is cold, then loosen the bleeder screws; and add a mixture of equal quantities of coolant and distilled water at the radiator filler neck, until coolant begins to flow out of the bleeder screws. When the coolant flows out in a steady stream and the bubbles and air pockets stop coming out; close the bleeder screws, then start the motor and fill the radiator to overflowing while the motor is cold and idling. Install the radiator cap before shutting the motor off, and then fill the coolant reservoir to the level line.
If pure coolant is used in the motor, without mixing it with distilled water; it will cause the motor to overheat. Also, Chevy radiators cannot be filled completely unless the motor is running (when it is cold).
The gaskets on the thermostat housing should be coated with a thin coat of gray or blue RTV silicone gasket sealer before installation. And the mating surfaces that the gasket touches must be cleaned of all traces of debris and old gasket material (preferably with a blade scraper) before installing a new gasket.