Please help me fix the infamous hissing dash on my 86 MGM

[TwiztidRat]

Hey guys my steering shifter on my 86 MGM has the infamous annoying hissing noise when shift into R or D. It stops when it's in Park. It's not the parking brake release from what I was told. A reputable garage tried pulling the lines and plugging them and it still was hissing. I listened under the dash and it sounds like the hissing is coming from around the main brake pedal, someone was telling me they think it's the vac line for the cruise control? I noticed when I use the CC it works, but it will drop speed every second and rev back up so I'm thinking it is affiliated with it. My question is what line goes to the cruise under the hood? Thanks guys.

[gadget73]

there are 3 things under the dash that typically make this noise

1) parking brake release. This is the only one that is likely to change based on what gear the car is in. The valve is the half moon looking thing right above the steering shaft at the firewall. Usual test is to plug the vacuum line to it. if the shop was trying to plug the line on the parking brake assembly that won't fix it. You can replace the valve or just leave it plugged, your call. None of mine work.
2) brake booster. This one usually only makes noise if you apply the brake, but it might hiss with no brake and stop with the brake. No relation to gear position.
3) stuff in the climate control. Also no relation to gear position, but might change depending what you do with the climate control

There is a vacuum dump valve for the cruise control, but the only time it has vacuum is when the cruise is actually being used. Definitely not the case in park. Leak at the dump valve can cause this surging problem though. Had it with one of mine. Its a fat hose, goes directly to the cruise servo. If you look at the cruise servo, there is one line that goes from the front section to the rear using a short U shaped hose. On the rear side there is a Y shaped fitting where it connects. The "other" hose on that Y fitting is to the vacuum dump valve above the brake pedal.

Make sure the dump valve is pushed all the way in when the brake pedal is up before getting too far along. If its not fully closed it can leak and cause stupid problems.

[TwiztidRat]

Thanks for the reply Gadget, so you are saying that the hissing wouldn't be affiliated with that cruise control line even the u shape one or whatever. As in you are stating that could be the problem with my cruise surging? Right now I'm just trying to stop this hissing, I just picked up this car last week and it needs some TLC of course being an 86 and wasn't cared for the best but right now I just want to shut that hissing noise up. If it's still possibly that parking brake I could careless about ever using it if I can still plug the line, they tried plugging the two lines on it, but I noticed in the firewall under the dash there is a big vacuum line it's not easy to get at or pull off but is that the main line I would just have to plug? I couldn't see what the shop guy was doing but maybe he didn't actually test that line part..

Also best way to describe the hiss how it reacts is if it's in Drive and you try to wiggle the lever to switch it back to N or R that's when the hissing almost becomes more higher pitched? I don't know how else to describe it. It's like the gear lever is plugging and unplugging the hose itself if it was leaking air out of there.

[79lincolnlover]

If you can't fix the hissing noise, turn up the radio (if it still works) or wear ear plugs.

[TwiztidRat]

At this point I might have to just tune it out. It'd be nice if I could stop it but I can't reach in and disconnect this one hose if it even is it. So last thing I want to do is create more problems.

[sly]

If it's not the dump valve for the cruise, check the cluster of hoses behind the ash tray to see if something has come loose on the vent actuators for the HVAC system. They sometimes slip off just a bit and snugging them back up solves the problem.

[TwiztidRat]

Sounds good Sly, I haven't ruled out the dump valve for the cruise though I feel stupid asking this but where is that thing located? I will check the hvac hoses but as far as I can see the system works like it should with the blend options and heat settings etc. I haven't checked the cruise part though. That's what I am hopefully assuming it is.

:EDIT:
Wait never mind I forgot Gadget pointed out where to look on the cruise valve thing. Thanks guys I'll check these out.

[gadget73]

hiss in park is probably the half moon shaped thing.

The vac dump valve itself lives above the brake pedal, but its easier to test it from under the hood at the cruise servo.

[TwiztidRat]

I'll have my buddy check out the servo, cause it only hisses in D, R, and N. Don't quote me in N but I believe it hisses too. It doesn't make the noise in park though so either way someone has to hold the brake down while I'm under the hood.

[gadget73]

the only time the cruise can possibly make noise is if you're using it. The dump valve has no vacuum on it until the cruise is being used. If its hissing when stopped, thats not your problem. Could be the surging, but its not the source of the noise.

[TwiztidRat]

Oh yeah my bad, well for now I give up it's gotta be the brake release. I'll take a picture of the line I am talking about trying to tamper with tomorrow cause I don't want to monkey with it and find out it's worse off but not sure what else to check except those hvac hoses but I checked them and they were all good as far as I could see.

[sly]

the horseshoe shaped valve controls that brake release valve with the white or grey line and the black line should be the source vacuum. Pull the line to the brake release and if the hissing stops when you plug the stem on the horseshoe valve, then it's the release actuator... if not, pull the source line and plug that and if the hissing stops, it's the horseshoe valve.