So after getting in my rig today at a local hardware shop I go to put it in reverse and back out. Once I was ready to put her in drive I went to hit the brakes for my gear shift and presto, stiff brake pedal. Damn near had to stand on the  pedal to stop it. After some reading I found out that my 96 2500 2wd Cummins has a “vaccume” type brake system for power boost. Looking at the lines on the fire wall I noticed one was popped off and it seemed to fix the brakes. Now Ive recently also had the issue of “zero” in cab air control. It’s defrost vents all the time, looking at that line it seemed loose so I zip tied it, I noticed I may have a hose missing. I think this is where the line for 4x4 guys goes to the transfer case, is mine a vent since it’s a 2wd?

Should be capped. If your just a 2WD truck then you should have a cap on the tee and nothing there.