05 Chevy MC coolant problems

First noticed a while back the sloshing sounds coming from under the dash & engine compartment & lack of proper heating ,1st. topped off coolant then bleed air seemed to have fixed it then about 2 months later the sounds came back then noticed coolant leak coming from water pump, diaphragm holes behind the pulley. With only 44,000 on the 3400 Ls getting ready to replace water pump but when taking the radiator cap off noticed brown sludge around inside of cap + same sludge inside radiator throat but none inside the water pump housing also what I drained from the lower hose looked OK. Wondering if I need to flush the radiator? I have heard that air alone can damage the pump is this true? I read where you need to remove the hose from the heat core to fill with coolant (like to prime it so as to min. the air) while filling the radiator keeping the engine revved to 1500 before replacing the main radiator cap does anyone know of this? I feel I either had the leak with the WP or a small leak somewhere else. Because it is definitely loosing coolant over a short time.