[red caddy]

Have you checked the equalizer ports in the doors? The louvers on the trailing edge of the door let air trapped in the cab escape thru the louvers in the inside door bottom. This port has a rubber "flapper valve" (one way air flow but keeps water out) on the inside of the door that will stick shut or just get hard with age and ozone.

Also, If you don't take the plastic block out of the doors,before you paint the doors, you can paint them shut or stiffen them to the point that the cab air cannot escape when you slam the door.

If you have AC, the outside air inlet ports in the footwell, usually vacuum opperated, can stick shut, (the normal position) and this will make the cab close to air tight. I've seen it so bad, that when you slam the door, your ears pop. When I restored my old '67 VW bug, the new seals were so tight that I had to crack open the wing vent to get the door to latch without slamming. RED