Chasing vacuum leak

[VPRob]

I've got my aircon fixed after pulling dash out, re-building heaterbox with new evaporator and heater core (I do not want to do this again!) and re-surfacing all the flaps, but I have ended up with a vacuum leak.
It sounds like the leak is at the switch block but I have had 3 different SBs in and they all sound the same and the leak is still there. When I switch ignition off I can hear it hiss until the vacuum is gone and it switches between vents while I drive.
My theory is that what I can hear at the SB is the air continuously rushing through it because of the leak and that is why it sounds like that is where the fault is. I suspect one of the actuators near the fan box is the culprit. I may yet have to pull the dash back from the firewall to get at it.
Any tips for establishing the actual source of the leak?

 

[strgas]

smoke generator and blow back into the vacuum lines .

 

[FXST01]

Pull the tubes off the SB and block them off. Do you still hear the hissing?

 

[VPRob]

Pull the tubes off the SB and block them off. Do you still hear the hissing?

words of wisdom from The Grey Man. Why didn't I think of that? Thanks.

 

[VPRob]

Turns out the vacuum leak was because one of the vacuum lines had come off its actuator. The resulting air rushing through the switch block made it sound like the block was faulty. It's really nice to have my aircon working. The VP feels luxurious again. It was a mongrel of a job though

 

[Harrycoop]

Turns out the vacuum leak was because one of the vacuum lines had come off its actuator. The resulting air rushing through the switch block made it sound like the block was faulty. It's really nice to have my aircon working. The VP feels luxurious again. It was a mongrel of a job though

VPRob, I realise this post is four years old, but did you have any luck fixing your vacuum leak? I have found the yellow vacuum tube from the heater switch floating behind my dash. Do you know where the yellow tube connects?
Cameron

 

[Comcal76]

 

[VPRob]

VPRob, I realise this post is four years old, but did you have any luck fixing your vacuum leak? I have found the yellow vacuum tube from the heater switch floating behind my dash. Do you know where the yellow tube connects?
Cameron

The vacuum hose had come off the actuator up behind the glovebox so once I realised that it was an easy fix. The air rushing through the switch block (selector) because the hose was disconnected made it sound like the fault was there in the selector. Remember I too am an amateur mech. From my (sketchy) memory I think the colour coding for your loose hose may be on the connection points on the back of the position selector. I don't remember whether or not you have to pull the selector out to access the back.
If you don't have this fixed ask for more help. If you have it fixed then please tell us what and how.