I got a vacuum leak code and the heat control module doesn't the heater door to let the hot air into the duct system. No vacuum hoses in the heating system are off. Car runs well, although there is an increasing hiss under the right dashboard whenever I step on the gas while driving and when I turn off the engine. I assume it is the vacuum reservoir discharging. Given the price of a control module (CA $350 used, CA $850 new), my mechanic has offered to convert the temperature control dial to a simple 3 way electrical switch to operate the heater door and bypass the vacuum system.

QUESTION: What other components on this car, besides water heater valve, are vacuum controlled? Are there any adverse effects of redoing the heater switch and ignoring the hissing as long as the car runs well?