Heater Valve Background
The heater tends to fail fully on or fully off.
The function of the heat sensor tube is to close the tap when exiting air gets too hot, using expansion of the fluid inside it, which is heated by the exiting air. Thus thermostatic control of the heater air is achieved.
The working fluid can leak fron this tube, and this regularly leads to an always on heater.
The SM heater has over closed capability. This means that as you go past the start of the white section towards the blue the valve is already closed, but the actuator, being mechanical, continues to travel in the same direction.
When left closed for very long periods, the tap diaphragm can become 'glued' to the sealing face. This effect leads to failure of the pull off cup inside the valve. As the sealing spring is quite strong, once failed, the pull off pin will not work again.
Recommendation: Store your SM with the heater valve open.
It is relatively easy to 'shim' the thermostatic element to achive closure of the tap but without thermostatic control. Repair of the always closed situation is much more difficult.
Lay on your back with a flashlight (torch) and determine if the valve is responding to the lever and linkage. If it is it is probably a faulty heat sensor tube (I think they are called borden tubes or something like that).
Cut the spring
If you reach into the heater valve spring cage with a very good pair of side cutters or nippers and cut the spring in one or two places the valve will revert to a manual heat control. In other words, collapse the spring. The spring material is extremely hard.
(or) Add a Spacer
Put a short slug made from a 5mm bolt into the head of the heater capsule and all is well. But you do have to take it apart, nearly a 5 minute job (really).