heater functions switching when vacuum drops

Anybody else having their heater output switching from floor to dash vents on it's own. I have learned that it was a common "problem" back in the days when our cars were new. I'm told that GM said the fix was to install a check valve in the vacuum line going to the heat and a/c controls. I think I'll try it.

The heater doors are vacuum controlled. loss of vacuum...loss of doors. Can't say I ever say a one way valve on any of the line though ('93)

The valve should be at the front of the TBI in a 91-93. It is a small black and white plastic part about an inch and a half from the port. The white end should face away from the TBI. I do not know if there is one on the LT1 cars, but I would suspect that there is.

I haven't seen one on my '91 TBI car and have access to all the tech bulletins that GM sent to dealerships but haven't seen a bulletin about it either. A GM tech told me about GM's fix being adding a check valve. I bought a valve at O'Reilly's yesterday to add to mine ( or replace the one already there). No big deal, I suppose, just a minor annoyance.