Here's the deal. I have a 97 blazer-jasper rebuild installed after purchase(I new when I bought it this was needed) new fuel spider done by Chevy, ever since I hear a hissing behind dash. Rear brakes tend to drag, sometimes doesn't like to go into 4x4, where is the leak?! I know it's there just can't find it. Have replaced several vacuum lines that were dry rotted under hood including the one that goes to the brake booster. Please help-ready to scream.

 

If hearing hissing from under the dash, sounds (no pun intended) like the leak is at the HVAC controls. Find the line under the hood that goes through the firewall...plug it under the hood and see if that stops the hissing sound.

If it does, then time to get under the dash and continue searching. Could be a vacuum line, the HVAC controller, a vacuum actuator...or all of them.

 

OK so today I picked up a mighty vac. Quite handy. Checked all lines coming into HVAC control, found them all to hold vacuum except the grey one. Went back under the hood, started checking lines around line feeding through fire wall and "ball". Found them all to be good except the one that goes to pcv. It will not hold a vacuum and with engine running has needle very unsteady. Normal? What next?