Help with a/c blowing out defrost ?????

On our 97 Blazer 4x4 the A/C blows out the defrost & floor. It started not blowing at all on hills. Then I replaced all the vacuum hoses under the hood. It worked OK for a few days then started blowing out the defrost & floor on hills. Then today it started ONLY blowing out the defrost & floor.
I'm aware it is a vacuum leak. But where under dash should I start looking? GM use to use a vacuum valve to change modes. Do they still? If so where is it?
Any help would be appreciated. Thanks in advance.

Where to start looking?-- at the vacuum source of course. Trace lines, pull apart each connection, see if vacuum exists- if not, then back up one connection, till you find it.

Hint: a 97 S-truck has vacuum reservoir ball mounted on hood as I recall- see if it is unhooked. Also check that checkvalve is flowing properly.

Thanks for the response. I will check the flow valve. But I replaced all the vacuum lines under the hood ie the ball & lines. From there where do the lines enter the dash. I found one on the drivers side it is OK. Is there a vacuum control where main vacuum enters & is directed from there to the mode doors, & if so where is it?

It's on the control head- but very rarely is your problem solved by looking inside the truck--It is almost always under the hood...Unless someone has been inside the dash installing electronics etc....

Thanks again, The only thing that was done was a heater core replaced but that was 2 years ago & the problem just came about. That is kind of a relief knowing it is most likely not under dash. I need to check that flow valve, I didn't get to that last night. The only other thing I can think of is the ball is cracked or leaking. The heat in that area melted or burned the vacuum hoses. We live in Yuma Az with hot temps.
Thanks again I'll keep you posted. I can't hear any vacuum leaks under dash. Well the search continues. Thank God the A/C does blow cold through the defrost & floor so @ least we are not without. We are headed to our summer cabin in two weeks so hopefully I can find it, if not I have all summer in cooler weather to look. Thanks

For anyone interested. I finally fixed the problem. What I found was the vacuum source line going to the transfer case switch was burned & rotted from heat. Also I found the vent hose burned & the hose from the T/C switch to the front axle actuator. I replaced & repaired the hoses. I now have my A/C/Heater vents working correct & I'm sure now after testing the T/C & front axle are working correctly. I thought my 4x4 was working but I don't see how the front axle could of locked in. I know the T/C was working & not sure how that was working either. One thing is if I ever have to completely replace the front axle actuator vacuum line I would just cut it & reroute. I still can't see were it runs but must enter into a loom.
Thanks to brcidd for pointing me in the right direction.
I hope if you ever need to do this fix you have more patience than I do. It is a B---- to get to these lines under the vehicle without a hoist or ramps.