Having read all of your comments, I have decided that I will be
taking the heater box
out of the car first in order to replace the heater core.
I'll have to order parts first, so it may take a while.
I'll let the list know how it went, once it's done.
Unfortunately, it means that I will have to remove the brake fluid
My car has a single hydraulic circuit, but with a remote transparent
reservoir VERY close to the heater box,
so that the brake fluid level is visible immediately after you open the hood.
I think it was a legal requirement, resulting in a feature unique to
cars delivered to this market (Holland, Europe).
Suggested annual donation $12.75