Vent/Defrost Issue

[cory99]

Ever since I owned this truck it has acted up and now that I need AC figured I better get it fixed. I have trouble getting it to switch over to vent, it works every time on defrost or floor, if I turn it to floor after a second or so you hear the blend door slam closed then you can try to turn it to vent/floor and once it switches to there you can then switch to just vent but you have to mess with it a few times to get there. I have read some threads where it was stuck on defrost but nothing really along these lines anyone got any ideas of where to start checking, maybe a vacuum leak?

 

[JLDickmon]

or a bad mode switch..

 

[Bobs 7.3]

mode switch of vacuum leak somewhere.