ac/heater control malfunction

[Poker]

1985 k5 ac/heater control modeul will not work right.
after looking into it... i see that the vaccume isnt activating to turn the vent to allow it to work. question is .. is it the vaccume or the ac/heater control... and how do i find out which it is?

the air blows and the motor works on low med med/high but not high.

thnx for the help

 

[tx_sub]

check the relay on the ac box under the hood. the same thing happened to me this past summer. 15 or so for the relay at napa. fixed my problem.