My rear heater core was leaking. When I disconnected it, I decided to replace
all of the heater hoses with new ones.  To maintain the capacity to reinstall
a new rear core, I mounted new "T"s in the lines to supply the rear core
(plastic, to avoid corrosion) and connected the "T"s which would supply the
rear core together with a short piece of hose.  To prevent the flow of
coolant through what would become a shortcut in the heater circuit and
degrade the performance of the front heater, I placed a marble (glass) in the
hose connecting the "T"s.  It was large enough to block the flow of coolant
and too large to pass through the "T"s which were connected to the heater
hoses.

Rico