why will my truck only run if my master brake booster vacuum line is not hooked up? If i do hook it up it will die instantly or if not running and i hook it up it wont start

 

I would think its idle related, rather than vacuum related. But there is a check valve at the booster that the hose attaches to. I used to think it was just an elbow/nipple. But all it does is suck and provide vac to the booster.

So I'm thinking IAC behind the throttle body. If its gunked up with carbon (which it does) then it needs a cleaning. I added a HOW TO on here years ago for that procedure. That's the sensor that controls idle and when the vac hose is detached, it adds a huge vac leak which raises idle and thus, keeps your engine running albeit at a higher rpm. Stop the leak and now IAC has to keep idle at preset 550rpm which it can't. Try cleaning first!

 

connect the hose then pinch it off with vice grips and two small pieces of wood like paint stir sticks.

start the engine and it should idle.

release the vice grips and if it dies your brake booster is leaking.