Coolant Flush

Hi - After putting the engine back into the car I'd taken the chance to flush the coolant with a garden hose.
I'd done this a year back - but this time I remembered to take out the plug on the front of the engine,
I was ready to see water flush out  - but it just about wept out a few drops.
I tried poking it with a wire and squirting from the hose in to it- no joy,

Should I add some chemical flush to the system , run the engine and open the plug when still warm?

Or just leave well alone?  

I imagine the cooling should be as good as it can - but I don't want leaks that have been sealed by time,

Paddy