I have 1996 f150 4x4 manual trans 302 eng. When I get on the freeway or pull a heavy load foot on the floor with the A/C on the air flow goes from the vents to the floor. It stays that way most of the time unless I fully let off the gas and coast. Some type of vacuum problem I assume. Engine runs fine no other promlems. No check engine light. Passed smog recently. Anyone have any ideas on where to start checking?

 

Sounds like you have a vacuum leak somewhere in the heater control system the first place I would check is the vacuum controler by the blower motor under the hood the white tube going to it is bad to get hard and crack there.
If not there you will just have to check the vacuum hoses going and coming from the heater control assembly.
And also you might want to check for leaks on the hoses on the engine and around the intake manifold itself.
These and the ones on the engine are easy to find by spraying starting fluid or carb cleaner on all hose connections and around the seals and gaskets on the manifold with the motor running and listen for the engine to pick up the fluid at the leak.
But with it passing the smog it's more than likely one of the hoses going to the heater controler.

 

Check for leaks at the vacuum reservoir too.

 

Like 91 said, you have a reservoir tank under the hood, looks like a coffee can with 2 small pipes sticking out one end. Some people confuse this with being part of the emissions system and disconnect it. Or it could have developed a leak and they took it out of the system. There should be a vacuum routing diagram under your hood to show you where it attaches to. Good luck.
And if you had a vavuum leak it would probable be idleing high and running rough.

 

Do not spray under your dash, too much electrical down there.

 

The 96 Vacuum reservoirs are plastic and oval shaped. Bit of an upgrade form the cofee can design lol.