Ford F150 - A/C or heater all blow out the floor vent

it has to be a vacuum leak some where, where should i look first? under hood, under dash? i just had a used engine installed so im betting they missed a vacuum line somewhere

I would start with vacuum hoses under the hood, particularly the pvc hose and hoses behind the battery. Then all the hoses on the engine.

So I start with the near the battery, but what do I do? Just inspect for cracks or to see if it disconected or do I try to pull a vacuum on the line?

Inspect for cracks, breaks and disconnects. On the pvc hose, particularly check the elbow.

Check your selector knob and make sure it is turning the stem, and not slipping on the stem.

Here is a link to a discussion and a vacuum motor schematic that might be helpful:

viewtopic.php?t=58092

ok after i took out the battery and the battery tray, there are 2 vacuum boxes attached to the batter tray. one of these had a major crack, i have epoxied the crack and waiting for it to dry. while waiting i have been using one of those hand held brake vacuum/bleeder things, but it seems every line i try doesnt hold a vacuum should this be the case? or do they not always have a vacuum?