It is difficult to adequately clean out the water jacket with the engine in the truck.
That petcock is at the bottom of the water jacket. Just cleaning the area right behind it will not be enough to solve an overheating problem.
You need to be able to clean out the bottom of the entire block. Most times there is nothing available to dissolve Stop Leak.
I had a real bad one with over a pound of black junk in it. The only way to clean it out was by completely tearing down the engine. Freeze plugs, water pump, head, petcock all needed to be removed in order to break this mess up into small enough to get out.
I used a steel rod down the block holes (which supply the head with coolant) to break up the junk not accessible from the freeze plug holes.
It was a chore to clean that block.
That black junk was likely several doses of stop leak added over a number of years to extend the life of the very expensive (back in the day) radiator.
