All these posts seem to have a common theme ...a suspected leak within the vacuum system. That being the case, the diagnostics involve checking all the vacuum hoses and connections for kinks, leaks, disconnects, etc. You might start with the vacuum connector(s) that go to the back of the Heater/AC Selector Switch on the dash and go from there.

Another thing to check (maybe even first thing) is the Vacuum Reservoir Tank, since its job is to help "moderate" the engine vacuum and to be a source of consistent vacuum to the subsystems, notwithstanding changes in engine rpm (and hence changes in manifold vacuum). It is noteworthy that the Reservoir Tank is the main (and sole) supply of vacuum to both the Heater/AC Selector Switch AND the PVH Vacuum Solenoids.

Ford says you can check the Vacuum Reservoir Tank as follows:

1. Apply 51 kPa (15 in-Hg) vacuum on the vacuum reservoir tank outlet port with a vacuum pump and observe the gauge reading.

2. If the vacuum loss exceeds 3.37 kPa (1 in-Hg) per minute, remove the vacuum pump and plug the vacuum hose (on the pump). Pull a vacuum with the pump to be certain that the hose and pump are not the cause of the leak.

3. If the A/C vacuum check valve loses more than 3.37 kPa (1 in-Hg) of vacuum in one minute, then the check valve in the tank is bad. The remedy is to replace the reservoir tank.