So I have reached an impasse. A couple weeks ago the brake booster on my 1995 SR failed and I got a suitable replacement swapped in without too much issue. A few days later, the truck dies on my wife, and when I re-start it, I hear a very clear vacuum leak. Nice loud hiss, but strangely hard to locate by sound. Spraying the various sprays under the hood has resulted in mixed results at best....

Here's where I am at. As best I can tell, the leak must be somewhere near the TB, but not AT the TB. It seems to be closer to the firewall, and possibly even under the intake plenum. I have felt every line I can as far back as I can and still have I come up empty. I have tried to look for anything helpful at all and I am still coming up short. What I do know is that the truck has trouble running at idle, and if I haven't stepped on the brakes in a while, it's like it needs to build up vacuum a bit before the booster really kicks in.

Any ideas on what is my most likely culprit is? Please tell me I don't have to remove the plenum, I really don't want to.....