A likely issue could be the vacuum supply to your HVAC door actuators. The vacuum hoses in the engine compartment supply vacuum to both the front axle on 4wd models, the transfer case (on the 3-button controlled 4wd systems), and the HVAC system door actuators. These will rot over time causing leaks. The rot can lead to slow leaks due to cracks, looseness at the hose barb fittings, or out-right disintegration of the hoses.

In my .sig file (below) I have a link to the thread I made on how I replaced the vacuum hoses on my 1999 Blazer with the 4-button 4wd system.

Be sure to report back what you find.

 

is their a way i can check if it is leaking or should i just replace it all? I watched a few videos and it talks about the actuators going bad.

 

is their a way i can check if it is leaking or should i just replace it all? I watched a few videos and it talks about the actuators going bad.

 

From what I've seen on the forum, the actuators are not likely to go bad although they can.

To replace the hoses is pretty simple and I would say is very much worth doing anyway because if they have not rotted yet, they will. What I am not familiar with are the three hoses that control the transfer case for the "3-button" systems. The 3-button systems have 2Hi, 4Hi and 4Lo whereas the 4-button ones have 2Hi, Auto 4Hi, 4Hi and 4Lo. The former are controlled by vacuum and the later is controlled electronically. I would suspect that the transfer case hoses would be if higher quality than the other vacuum hoses so hopefully any leaks will not be in them.

The thing about the vacuum hoses is that if you do have a leak in the system, the front axle will not engage for 4wd even if you selected it with the 4wd switch. So that is another reason to check on the vacuum hoses.

The nice thing about the vacuum hose replacement is, unless you have a 3-button system, you can do everything from top of the engine compartment.