AC gos to the dash wen u hit the gas
 

I have a 97 blazer i dont know whats whorng with the AC u put the AC to the front vents but wen u driving and u give more gas the AC go to the uper wind shield and u stop hiting the gas it go back the front vents can some one help me.

Most of the dash vents are controlled by the vacuum. You likely have a problem with the vacuum reservoir located under the hood (round ball).

so have to replace the round ball or check all the lines

Checking the vacuum system under the hood would be a good start...

i check all the lines and they look ok and the round ball looks good they are no crak in the lines whats the next step.

Well, you have a leak in the system or a system that is not properly plumbed. You either have to find the leak, or plumb the system properly to fix your condition.

i just bout the blazer and the guy put a new transmission and i know the transfercase has a vacuum line i dont know if that can be the problem and the line go to the firewall is a small line and is a T it can be that.

check valve
 

had this problem on 2 blazers/jimmy. behind motor, there was a hard vac line that went to a small one way check valve, the check valve is plastic and one of the nipples where the vac line connects was broken off. I think it was like 15 bucks at dealer, although i've seen one just like it in the "HELP" section at local autoparts. The vac line was behind the motor near the firewall, can't remember which side i'd say in middle or towards the passenger side...IF you see the little checkvalve it's possible the nipple has split/cracked but not yet fully fallen off.

haveing the same problem in my 95 blazer. my rubber hose is is cracked but no hole and looks like the is pinched like sucked in but when i take off the rubber tube/hose is goes back to normal untill i start the blazer. if i replace the rubber hose/tube will the hose be fixed. Can i just get any rubber hose that the same size/ lenght or do i need a certain hose?

Any typical vacuum hose of the same size will work.