I have a 92 F150 with a 5.8. My air conditioner won't stay blowing out of the vents it switches to the defrost vents. I can make it come back to the vents by downshifting or raising the engine rpm. I found the air solenoid that controls the air flow to the vents and the line that hooks to it . It seems to have very low vacuum pressure. I traced that line out under the hood and then to a block in the intake manifold. After that I am lost as to where to look for the problem.Any help would be appreciated...

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1992 F150 SC 4WD,5.8L,E40d, 180k miles

 

On mine, the vacuum path is from manifold, to white plastic check valve near A/C evap. under hood. The other side of the check valve has two plastic mini hoses. One to the vac. res. on side of evap. case, the other into the harness and into interior. There it goes to the selector panel. In A/C, I think the hose you are interested in is the Blue hose, it goes from selector to the panel/defrost vac motor. The purpose of the check valve and vac res. is to trap usable vac. into the system, to carry over during times of low vacuum, like accel under load, when throttle plate is open pretty wide (intake vac low). If check valve leaks back, or if vac res. or hose connections leak, or leaky selector, or leaky diaphram, then there's problems. Probably want to test your way along. If you have a vac gauge, can check vac and check valve by putting gauge in instead of vac feed to inside truck, then run it on road. Can keep moving the gauge closer to the diaphram to find problem.
A hand Vac pump/tester can test most of it without engine running.