94 b250 AC

ac works good on the norm setting but not on max it will blow hard for 30 seconds then its like a door closes and shuts the air flow off

There is a return vent on the passenger floor that is used when on Max AC...is it covered?

no sir I don't think so. the big vent in front of passengers feet is open

Well to me it sounds like something is clogging up that interior air return. When you switch to Max AC it closes off the outside air so that air is now mostly pulled from in the cabin. It would seem the duct for the outside air is passing air just fine which is why I'm focusing on that interior return air duct.

yesterday while driving I switched from norm to max ac and it was working until I had to accelerate then it shut off like before ill try and see what could be blocking it thanks

Mode door movement while accelerating is most definitely a vacuum leak. 99% of time vacuum leaks are under hood at vacuum line source- usually rotted rubber or plastic tubing at intake manifold. trace your vacuum lines and determine where the problem is, and replace as necessary- Engine heat kills rubber and plastic lines over the years...