remember before the dial or the short sliders, cars had the long side to side hot/ cold, or a mix of lever, there was a cable off the back that lead to a valve, when cold you were closing the valve, so it blocked off the core, hot opened the valve to let coolant flow through, why back in the day you were told to put the heat on full, the fan never did a thing either, just had to open the valve. aka to hot.
cant tell you how many of them i had to change, they used to fail when they got old and leaked all over the place.
today we get blend doors.
