On my 63 Morris Cooper S I have an "upgraded" heater labeled British Leyland,
so newer than the car. it has the proper heater cable operated valve on block.
However, when I shut off the valve ( I freed up the cable and valve connections) I still get heat through the core. I would like to shut the core off in the warmer months (car runs cool enough I think) and wonder if a small brass shutoff valve in the return hose would make that happen and if there is a downside to that (or other ways to do this). Any suggestions would be appreciated.