find out how the orignal cars heater matrix was controled (air flap or valve)
some require flow all the time trough the matrix to get the same cooling(ballanced) to all the cyl's
the way i would do it and works is block off if it has a valve controling the coolent flow if not link the input to the output and place somthing simple like a washer or thiner pipe to ristrict the flow to simulate the matrix's flow rate
thinking about it this would work for both types and would remove the risk of air pockets
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