The image in post #6 is taken from the center of the engine looking at the left inner fender. The part on the right is the ABS unit, and the part on the left is the cruise control module. It's possible that the plastic nipple is broken off flush with the vacuum reservoir, in which case, you won't feel it.

This image show the orientation of the reservoir, (the arrow points to the front of the vehicle):


Here is another view, (the arrow points to the front of the vehicle):

 

Thanks, I'll try again tomorrow.

 

So on non 4wd Blazers, is one of the ports on the side of the check valve that has 2 nipples (lol) just capped off? Or does it go to something else?

 

On 4WD there is an additional vacuum Tee with a line that runs down to the vacuum switch on the transfer case.

 

Would the nipple be located in the same location on my 2004 blazer?

 

AFAIK, Yes

 

I found the vacuum res but the nipple is broke off. How do I fix that?

 

The nipple on the vacuum reservoirs is recessed. Have you confirmed this?

If the nipple is broken, I suppose you could epoxy in a plastic tube or try to replace the reservoir. I've attached the instructions for replacing the reservoir but it requires removing part of the braking system (also attached).

The part number for the reservoir looks to be 15720833
(https://www.gmpartsgiant.com/parts/g...tra2=&filter=())

The final solution is to find a vacuum reservoir that can be mounted somewhere else. The 1st gen Blazers had one that was attached to the inside of a fender or to the underside of the hood I think.

https://www.dormanproducts.com/p-17836-47076.aspx