I have a 71 Toronado and have had to add a vacuum pump because of the cam im running has caused low vacuum so my power brakes were not function.Soon as I add the vacuum pump my brakes started working fine but it has caused a hissing noise behind my A/C controls in the dash.It will only stop when I slide the colder/warmer control lever all the way on cold.Just wanted to know why it is doing this.It was not doing this before the addition of the vacuum pump.Is it possible there is to much vacuum?I just have put the car on the road a few weeks ago ofter the motor rebuild and do not have the compressor for the AC lines hooked up.I would really appreciate your help on this. Thanks Roy

 

Likely you've a vac leak in the dash. Maybe your low vac issue wasn't entire due to the cam, but because of a leaking diaphragm. Slapping a high vac source into the mix may have blown that leak wide open.

 

Yes, sounds like a leak in the valve or the lines that control the hot water valve.
I am not 100% certain of the function in each year, but it sounds like your valve closes when vacuum is not applied, and opens when vacuum is applied, thus, when your Temp control is anywhere but "full cold," the control valve to the water valve is open, and vacuum is leaking, instead of being transmitted to the hot water valve, as it should be.

- Eric

 

Thanks for the advice.I will take it apart and let you know what I find.Seems there are several lines that are going to the controller.?

 

Yes there are. Check the diagram in your Chassis Service Manual.

- Eric