ello, I have a 1980 deville with a 368,I would like to flush cooling system,I have heard if you take off small hose near top of rad right behind the filler hole,you can put a garden hose into it ( the small hose) and this will do the job, what about the thermostat,will it stop the flow getting into the block?, would I also open petcock drain at same tme?. tnx