Originally Posted by GraphiteGhost

YES, you CAN bypass the core. I would suggest you get a "Y" and install it rearward of the duovalves (leaving the two exiting the firewall loose/open), and then tying the final leg of the "Y" over to the supply line that goes from the engine TO the core (leaving that last single heater core line open at the firewall). Make sure the system is drained below the lowest open line before you try to bypass the core at this point or as soon as you cut into those lines, some coolant will leak out.This will isolate the three lines connected to the heater core (sealing the entire cooling system up). The alternative is just CAP all three, and stop all flow at the duovalves (returns) and the other side at that supply line. IF you just cap all three, remember to cap BOTH duovalves rearward side on both valves AND that supply line going to the core (single line) on the opposite side of the engine. You do not want to let the duovalves get dried out (keep them wet) because if those valves aren't left wet you might have trouble with them after the core is replaced.


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Originally Posted by onehundred80

I cannot see why you cannot plug and clamp the single hose going into the duo valves, it should be easy as you have over a foot of hose free to get at. and do the same to the hose coming from the firewall by the power brake booster also with ample hose to access. All this after lowering the coolant level below the open end of the hoses.

It may be a good time to drain and flush the coolant anyway as the wrong coolant could have been used at one time and caused the problem in the first place.

You could if you felt like it join the two ends of these hoses to each other, with a hose extension using two barbed fittings and four clamps. Plugging both ends would be simpler and act just like the valves were closed.
The duo valve could be opened, cleaned and stored indefinitely.