[Healeys] Clean out cooling water passages

[Mitch]

Hello,
B 
I have all the hoses ,petcocks, radiator off.....................when I
drained the anti freeze looked good .......................but at the petcock
for the heater /shutoffB  ..when I removed I notice some rust/brown gunk in
that passage ..valve has been OFF for some time...........my question is
....is there a product to pour in to the cooling passages to clean the rust
etc. and then flush out with water some how?B  I do not want to damage the
water pump etc.
B 
Mitch
bj8