So there's a vacuum line from the intake to the brake booster, which has a "y" fitting, the vacuum line comes out of that and through a check valve then continues along the firewall for 10". It then has a "t" and another check valve and the line goes through the firewall between the heater lines. The line continues across the firewall where it "t's" again and goes through the firewall again and to the cruise control.

I'm pointing at the second check valve, this one leads into the firewall at the heater lines, the first check valve is in the foreground and in better focus at the brake booster. As you can see, the wire loom in the bottom left is in perfect focus.... Stupid camera!


I have a vacuum sorce but no way to control it, so, maybe just replacing the check valves would be an easy test to see if the problem is under the hood or under the dash... Couple of these for instance $4.99 at my local autozone.

 

Well it's out of focus but I think I see the line in question. In the pic your birdie finger is pointing to it. Follow that line down and see if gets smaller and goes into a thin harder black hose and that hose goes into the cab where the heater core lines are. If so then it could be that line leaking. If you find the crack you can place rubber line that is a size just up from it over to act as a sleeve.


Look really close at all the lines for cracks. There is only a few.

 

Well, I finally got around to trying the spraying of the various fittings in the engine compartment with starter fluid. I worked through everything I could think of-- from the master brake pump, across, the air intake, whatever appeared to be vacuum related, I sprayed it. If I thought it could/might be, it got sprayed.

Nothing.

The sound appears to be more and more located in the center of the engine compartment.
Granted, with the engine running, it's really hard to tell with any certainty.
When I close the hood, the hissing sound is as obvious as a sun on a clear day at high noon.
Of course, that does not tell me where it's coming from.

 

if its an evap leak you might need to have it smoke tested to find the leak, im in the process of designing my own backyard smoke testing machine, if i get it finished and working i will be sharing the plans on here

 

Don't use starting fluid on the engine. It is highly flammable and WILL cause a fire. Carb cleaner isn't nearly as volatile and can be used to find a vacuum leak. did you spray down the intake. everything behind the throttle plate to the intake valves are under vacuum except for at WOT.