'92-96 HVAC Vacuum System
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Others '80-91 similar, except '87-early '88 which use a cable for Floor/Def/Panel. The '80-86 vacuum reservoir is a plastic ball on the R wheelwell.
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This is the most common failure of this vacuum system:
It can be permanently remedied by replacing the underhood vacuum lines with silicone:
The next-most-common problem is objects (especially pens) falling into the defrost register and binding the doors. Remove the heater core for access.
'87-93 trucks are known to have problems with the reservoir on the evaporator cover warping the cover until it cracks, or the sonic weld attaching them cracks; particularly if the shiny insulation is missing, exposing the reservoir to exhaust manifold heat.
A problem for '87-91s is the temperature cable coming out of its adjusting clip:
'92-94 trucks with a single push-pull temperature cable should be upgraded to the revised pull-pull (looped, apparently double) cable & control panel.
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The vacuum check valve at the top is
Motorcraft YG-193-C 
