Vacuum issue - 1991 Grand Marquis (Huge Surprise!

[Duke Napalm]

Hey guys, I have (what I'm hoping will be) a quick question with an easy solution!

Before I ask, I will say that I know the mysterious "vacuum leak behind dash" issue has been extensively covered, at least for the 89 on down (from what I've seen anyway, read lots of old threads on the subject). Auto parking brake release, yadda yadda. On those, it was a little horseshoe reservoir right against the firewall with two easy to see vacuum hoses. I was bored tonight so I pulled the lower section of the dash apart and for the life of me I cannot tell where the hell the leak is coming from. The hiss is pretty loud while the car is being driven, but is only very noticeable at lower rpms. When you turn the car off, the hiss continues for a couple seconds as it bleeds the pressure out from the source.

Now, the only vac lines I see are a black, grey, and larger tan one coming out of the firewall right above the gas pedal. I plugged the tan one, as it snakes way up behind the steering column into the dash. But the hiss is still there! I believe the other two are for the heating/cooling system and I'm pretty sure those are okay. Am I missing a line or two somewhere? Tried spraying some carb cleaner around, but no dice.

Thoughts?

Thank you!

[mcninetyone]

When the brake booster is going bad it will hiss vacuum at the brake pedal connection.
You can tell if it's the booster because putting on the brakes stops the hiss or changes the pitch.
You have to listen close.
I've seen a lot of cars with this problem, including mine.

Or it could be a leak at the back of the climate controls where all the vacuum lines meet.

[Duke Napalm]

It does actually change in pitch when I depress the brake pedal, though I just assumed that came with the changing pressure in the system and had nothing to do with the brakes. Is this a genuine leak? Can I stop it somehow(short of buying a new one)? Where, hypothetically, is it hissing? Because it seriously sounds like it's coming from inside the car and not from the engine bay. Or perhaps I'm just missing something. Or insane.

Edit; I'm not sure if it matters but the car stops on a dime, and when I'm driving you can't really hear it all that much. Even when I'm coming to a stop.

Thanks

[gadget73]

Shut the car off and wait for the hiss to stop. See if you have brake assist still. It should be good for a pump or two before losing all the vacuum. If it has nothing, the booster is probably shot.

[Duke Napalm]

I think the booster is fine, even when the car shuts off the brakes have full pressure. There isn't any give at all in the pedal, even when I let it sit for a while. Isn't there some kind of vacuum reservoir up in the dash though?

[mcninetyone]

The brakes will work just fine with this vacuum leak, and they will work like that a long time. My car did this for several years before I got sick of the hiss and installed a new booster. That is the only way to fix it.
This is a brake booster vacuum leak, very common problem. If the pitch changes when you brake, 99.9 % sure it's the booster.

I don't know of any vacuum reservoir in the dash of a 91. And I've taken my dash apart several times, never found one.

[Duke Napalm]

Alrighty, after reading your responses and further examination, I'm not very concerned and am almost sure it is indeed the brake booster after all.

Thanks guys!