1975 GMC Pickup Heater Switch

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On our 1975 GMC Pickup truck when we turn the engine on the heater blower blows all the time, and we can't find a way to switch it off. Can anyone help us? The lever on the heater works on high, medium and low, but won't switch off. Is there, or should there be a separate switch to turn off the blower?
Also for anyone trying the bleed the brakes after fitting a brake master cylinder. I couldn't get a pedal no matter how many times I bled the brakes. In the end I opened the back bleed nipple and the front bleed nipple and bled them both at the same time, and I got a brake pedal. Hope that may be of help to somebody.