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1968 Vacuum Leak

Started by Sconnors, August 06, 2017, 02:29:34 PM

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Sconnors

When I put my 1968 Cadillac in drive I hear a hissing sound under the dash like a vacuum leak. It doesn't do it all the time but it is intermittent.  Someone said it could be the neutral safety switch. I took a bunch of pictures both under the hood and under the dash and everything seems to be connected correctly. Any help would be greatly appreciated.
Thanks
Scott
Scott Connors

TJ Hopland

4 vacuum things you could be hearing inside.   

-Something from the heating and AC system.   That could easily be intermittent because its operation changes depending on ambient temps and control settings.    Its a complicated system so troubleshooting could be complex.    There can be 'leak' sounds during normal operation but none of them should last more than maybe 10 seconds or so as it changes modes. 

-parking brake release which goes through the part neutral switch.   The valve is supposed to be off in P and N then open to hold the release open in R D 2 1.   This should not normally make any noises.

-cruise control.   The vacuum operated cruise control systems have a single vacuum hose plumbed down to the brake pedal that is an emergency 'dump' valve.   Its just teed into the line going to the servo that tugs on the throttle.   Brake pedal released this line gets plugged so the cruse could operate if needed.   As soon as you press on the pedal a valve opens so if the cruise was trying to operate its released.   Normally you don't hear anything here because its also electrically releases at the same time so if there is a slight woosh its split between at the pedal and under the hood so you don't hear it.   This should not be a factor at all unless you are trying to use the cruise.  If this was an issue I would expect the cruise not to operate very well or at all. 

-brake booster.   Its vacuum operated.  With age and miles they can leak.   Its more common to hear the leak when you are on the brakes but I have occasionally had them hiss at rest too. 
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

DeVille68

the neutral safety switch can make a hissing "noise" even though all the vacuum lines are properly connected.
There is a tap inside that moves a valve to actuate the parking brake. You adjust this unit with the two screws on either side to move it left or right. If the A/C is running the switch is also routing vacuum to the idle speed up. The line should be black with white strip.

If you hear a hissing sound the reason might be a miss aligned neutral safety switch.
1968 Cadillac DeVille Convertible (silver pine green)

James Landi

Hi Scott,

All of the suggestions are good --- is any of the before mentioned items not functioning properly?    DOes your automatic parking brake release?  Do you hear a CHANGE in the hissing was you move the shift lever???? (that's the neutral switch-- likely misaligned as suggested)   If not, how about the function lever on your A/C?
Does that hissing modulate as you move it?   There;s a mention of a brake booster, and here again, is there a change in the hissing noise as you push on the brake??? ALSO, and very importantly, you'll notice a "repurposed" #10 tin can that is painted black on your fire wall. (looks like a large tomato can) THIS MAY BE YOUR PROBLEM.  It is there as a vacuum reserve to make you vacuum door locks (and for '67 Eldo's "eye lids) operate AFTER the engine is turned off.  The tin can often develops leaks caused by the intense flexing from your engine generating appropriate vacuum in that can.  If you look carefully at the "Top and Bottom" of the tin can, you may see a split at one of the seams. 

Sconnors

Thanks for the great input. So I drove the car tonight and here are the results:
1. The parking brake and release work properly
2. The hissing noise only happens when I shift into Drive. There is no noise in Revers or Neutral, only when it's in Drive.
3. The noise does not go away when I step on the brake or let off the brake
4. The noise sometimes happens when I move the lever from Vent to Off and the noise continues in the Off position but stops when I put it in the Vent position.
Any thoughts after this information?
Thanks
Scott
Scott Connors

DeVille68

Interesting.
So, the idle speed up device receives vacuum only in P and N. The neutral safety switch seals the port connected to the A/C head in the other positions. If you hear the noise coming from down low of the steering column the cause might be this switch.

Otherwise, I suspect that the rotary valve is a little dirty or warped. If your rotary valve leaks from port 1 (vacuum source) to a nearby port, it could then cause vacuum from the power servo rotary to leak back to the rotary in the head (because in vent and off mode the power servo should be in full cooling mode, port 4,3,2 should have vacuum)

This post in my restoration blog http://forums.cadillaclasalleclub.org/index.php?topic=130360.msg348510#msg348510 can give you some ideas how this valve looks like. Below is also a post showing how to restore the vacuum canister.
Cadillac Tim suggests to use a tiny screw to hold the two halves together after you polish the surface. Make sure that you use some fine sandpaper  (I used 3000 and 5000 grid in the end). Actually, I used wet sandpaper with some WD40 for lubrication. Make sure that you put the paper on an old piece of glass or something very smooth and flat, otherwise you will not be able to sand the halves smooth!

You could unhook the small vacuum hose at the end of the vacuum tee on the firewall that goes to the A/C unit. If the hissing stops, then you know that I must be related to the idle speed up or the rotary valve itself. If not you could unplug the large hose going to the power servo. If it stops you know that the hissing must be related to some A/C component.
That is the brake, parking release, vacuum can, door and trunk release should be fine.

Best regards,
Nicolas
1968 Cadillac DeVille Convertible (silver pine green)

Sconnors

Nicholas
Thanks for the detailed information!
I believe it's coming from down low on the steering column, my next step is to have someone put it in drive with their foot on the brake and see if I can locate it. Question is shouldn't I be able to feel to vaccum leak by running my hands around?
I'll do some more checking and keep you posted
Thanks
Scott
Scott Connors