I have a 78 280z with no heat i tracked it down to the vacuum water ****. i have found that the vacuum is only sufficient to open the valve part of the way. i back tracked the vacuum lines to the Vacuum tank and cant find any leaky connections or hoses i checked my vacuum tank as well and it holds vacuum. i am not sure where to look next. any input would be very much appreciated.

Just a suggestion......your vacuum operated valve just might be worn out, causing additional resistance to actuate valve.

Tom

i found a cupling at the magnet valve that leaked when the hose was in the right way (the same way as when the vacuum tank is connected)i replaced it and the vacuum is at 19psi at the valve. the valve is working soall i have to do is put back the valve and see if the heat work. thanks for the suggestion.