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78 AC vacuum leak [message #312709]

Wed, 01 February 2017 07:19

C Boyd

Messages: 2629
Registered: April 2006

Karma: 18

Senior Member

I am working on a 78 PB that has a vacuum leak and will not switch air flow from floor to vents or defrost. I can pull the vacuum connectors and apply vacuum to each diaphram and all work and hold vacuum except the blue hose going to defrost door. The defrost door has 2 hoses going in the heater box from the drivers side beside the brake booster. The book says the blue stripe hose goes to the defrost door and the yellow stripe hose goes to the defrost door bleed. This is suppose to delay the opening of the defrost door and regulates the door in one of 3 positions. The yellow hose will hold vacuum but blue hose will not. I cannot find a way to get to the defrost door vacuum module. The control panel will not hold vacuum in any position. There is also a separate vacuum switch for the heater water shutoff. The shutoff valve will hold vacuum if applied directly. I am thinking the valves are leaking. Is the valves or the rubber wafers available? How can I get to the defrost door diapharam to check. Any ideas or suggestions appreciated.
Thanks..

C. Boyd
76 Crestmont
East Tennessee

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Re: [GMCnet] 78 AC vacuum leak [message #312716 is a reply to message #312709]

Wed, 01 February 2017 10:29

Emery Stora

Messages: 959
Registered: January 2011

Karma: 4

Senior Member

Most of the problems are with the little rubber wafer leaking. I had this problem several years back and I used a piece of plate glass with valve grinding compound on it. I rubbed the wafer (both sides) on the grinding compound until it was smooth, reinstalled it and had not had any more problems with it for over ten years now. You cannot buy a new diagram and it must have the correct grooves for the GMC Motorhome.

You would have to dissemble much of the dash to get at some of the vacuum diaphragms.

Emery Stora
77 Kingsley
Frederick, CO

> On Feb 1, 2017, at 6:19 AM, Charles Boyd wrote:
>
> I am working on a 78 PB that has a vacuum leak and will not switch air flow from floor to vents or defrost. I can pull the vacuum connectors and
> apply vacuum to each diaphragm and all work and hold vacuum except the blue hose going to defrost door. The defrost door has 2 hoses going in the
> heater box from the drivers side beside the brake booster. The book says the blue stripe hose goes to the defrost door and the yellow stripe hose
> goes to the defrost door bleed. This is suppose to delay the opening of the defrost door and regulates the door in one of 3 positions. The yellow
> hose will hold vacuum but blue hose will not. I cannot find a way to get to the defrost door vacuum module. The control panel will not hold vacuum
> in any position. There is also a separate vacuum switch for the heater water shutoff. The shutoff valve will hold vacuum if applied directly. I am
> thinking the valves are leaking. Is the valves or the rubber wafers available? How can I get to the defrost door diapharam to check. Any ideas or
> suggestions appreciated.
> Thanks..
> --
> C. Boyd
> 76 Crestmont
> East Tennessee
>
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Re: 78 AC vacuum leak [message #312719 is a reply to message #312709]

Wed, 01 February 2017 11:47

A Hamilto

Messages: 4508
Registered: April 2011

Karma: 39

Senior Member

C Boyd wrote on Wed, 01 February 2017 07:19

...I am thinking the valves are leaking. Is the valves or the rubber wafers available? How can I get to the defrost door diapharam to check. Any ideas or suggestions appreciated.
Thanks..

The rubber wafers are not available. As Emery suggested, the assembly can be cleaned and something like a tiny amount of grease might get a better seal.

Is the defrost door diaphragm inside the blower box? I am not familiar with the 78 model heater/AC blower box, but it seems to me the front cover comes off allowing access to everything inside:
http://www.gmcmhphotos.com/photos/dash-air-conditioning-redesign/p30764-slide141.html

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Re: 78 AC vacuum leak [message #312722 is a reply to message #312709]

Wed, 01 February 2017 13:42

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

Ace Hardware has a Plumbers Lube in a little tube. Safe on rubber. A very light film on the wafer contact surfaces helps it slide and seal. Also, GM used quite a bit of white grease on the sliders and cams of the control from the factory which needs to be cleaned off and refreshed after 40 years. I know that on GM the lack of vacuum or venting vacuum puts the system in "defrost " as a failsafe. If your wafer is leaking it will probably be in def. Not so sure your def motor is bad. It needs to vent to atmosphere for the springs to put it in Def. Unlike some expensive European cars (junk) those vacuum motors are pretty reliable on 70-80s GM.

John Lebetski
Woodstock, IL
77 Eleganza II

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Re: 78 AC vacuum leak [message #312739 is a reply to message #312722]

Wed, 01 February 2017 20:20

C Boyd

Messages: 2629
Registered: April 2006

Karma: 18

Senior Member

Thank you gentlemen. I tested both vacuum switches with ports blocked off and neither will hold vacuum. I do have a set out of a broken panel that the wafers look good. I'll try the grinding compound and see if I can get them working then swap them out. Water valve is easy, it's the 9 port that has the hold down screw coming up from the bottom. I'm hoping I can get to the defrost door actuator thru the glovebox if I need to. I'll get the valves working first and see. I had planned on using tire mounting lube on the wafers. I'm not sure how long it would hang in there tho? I'm not familiar with plumbers lube. I have some silicone grease that came with some urethane bushings. Judging on how long it takes to get off your hands and tools I may use it. Thoughts?

C. Boyd
76 Crestmont
East Tennessee

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Re: [GMCnet] 78 AC vacuum leak [message #312740 is a reply to message #312739]

Wed, 01 February 2017 20:29

Emery Stora

Messages: 959
Registered: January 2011

Karma: 4

Senior Member

If you can find it, a grease for desiccant or vacuum jars in a lab works very well. I comes in very small tubes.

Emery Stora
> On Feb 1, 2017, at 7:20 PM, Charles Boyd wrote:
>
> Thank you gentlemen. I tested both vacuum switches with ports blocked off and neither will hold vacuum. I do have a set out of a broken panel that
> the wafers look good. I'll try the grinding compound and see if I can get them working then swap them out. Water valve is easy, it's the 9 port that
> has the hold down screw coming up from the bottom. I'm hoping I can get to the defrost door actuator thru the glovebox if I need to. I'll get the
> valves working first and see. I had planned on using tire mounting lube on the wafers. I'm not sure how long it would hang in there tho? I'm not
> familiar with plumbers lube. I have some silicone grease that came with some urethane bushings. Judging on how long it takes to get off your hands
> and tools I may use it. Thoughts?
> --
> C. Boyd
> 76 Crestmont
> East Tennessee
>
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> Unsubscribe or Change List Options:
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Re: 78 AC vacuum leak [message #312755 is a reply to message #312739]

Thu, 02 February 2017 08:15

Bullitthead

Messages: 1411
Registered: November 2013

Karma: 5

Senior Member

The tire lube is probably water-based thick soap and will dry out, so I would use that silicone grease on the wafer. Smooth it out like Emery said, and/or possibly swell and soften it up a little with some Walmart carb cleaner or lacquer thinner (CAREFULLY, do not over-expose it) before you lube it.

Terry Kelpien ASE Master Technician 73 Glacier 260 Smithfield, Va.

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Re: 78 AC vacuum leak [message #312757 is a reply to message #312739]

Thu, 02 February 2017 09:15

Ken Burton

Messages: 10030
Registered: January 2004
Location: Hebron, Indiana

Karma: 10

Senior Member

Just a word of warning on silicone grease. One time I had a radiator cap that was very difficult to turn. So I put a light coating of silicone grease on the flat gasket inside the cap and re-installed it. Several months later when I tried to remove it, I could not get it off. I ended using a pipe wrench to turn it. The gasket part of the cap had swollen so bad that I could not get the cap off with normal hand torque.

Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

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Re: [GMCnet] 78 AC vacuum leak [message #312759 is a reply to message #312757]

Thu, 02 February 2017 09:29

emerystora

Messages: 4442
Registered: January 2004

Karma: 13

Senior Member

I doubt that the silicone would cause rubber swelling but it was probably incorporated into a petroleum based grease that would swell rubber.

Another warning on the use of silicone spray or silicone grease is that if you have an oxygen sensor in your exhaust, extremely small amounts of silicone can ruin the sensor. It will be drawn into the engine by vacuum and go through the combustion in the piston and into the exhaust. Anyone with tbi or fuel injection will have an 02 sensor.

Emery Stora
77 Kingsley
Frederick, CO

> On Feb 2, 2017, at 8:15 AM, Ken Burton wrote:
>
> Just a word of warning on silicone grease. One time I had a radiator cap that was very difficult to turn. So I put a light coating of silicone
> grease on the flat gasket inside the cap and re-installed it. Several months later when I tried to remove it, I could not get it off. I ended using
> a pipe wrench to turn it. The gasket part of the cap had swollen so bad that I could not get the cap off with normal hand torque.
> --
> Ken Burton - N9KB
> 76 Palm Beach
> Hebron, Indiana
>
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Re: [GMCnet] 78 AC vacuum leak [message #312766 is a reply to message #312759]

Thu, 02 February 2017 14:45

Ken Burton

Messages: 10030
Registered: January 2004
Location: Hebron, Indiana

Karma: 10

Senior Member

Emery,

Here is the stuff that I used out of an 8 OZ tube:

http://www.silchem.com/pdf%20Files/SM7704_UG-TechDataSheet_v10044.pdf

Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

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Re: [GMCnet] 78 AC vacuum leak [message #312767 is a reply to message #312716]

Thu, 02 February 2017 15:03

GMCWiperMan

Messages: 1248
Registered: December 2007

Karma: 1

Senior Member

NO! Death to O2 sensors if ever installed.

Ken H.

On Feb 2, 2017 3:47 PM, "Ken Burton" wrote:

Emery,

Here is the stuff that I used out of an 8 OZ tube:

http://www.silchem.com/pdf%20Files/SM7704_UG-TechDataSheet_v10044.pdf

--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

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Re: 78 AC vacuum leak [message #312772 is a reply to message #312709]

Thu, 02 February 2017 20:24

JohnL455

Messages: 4447
Registered: October 2006
Location: Woodstock, IL

Karma: 12

Senior Member

The Ace plumbers grease is for faucet O rings and gaskets and moving parts hence rubber safe and non toxic it's $4 for 1oz tube good for several lifetimes. I wouldn't get anywhere near the rubber or plastic with thinners or carb cleaners.

John Lebetski
Woodstock, IL
77 Eleganza II

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Re: 78 AC vacuum leak [message #312773 is a reply to message #312709]

Thu, 02 February 2017 21:06

Adrien G.

Messages: 474
Registered: May 2008
Location: Burns Flat, OK 73624

Karma: 1

Senior Member

Charles, PM sent.

Adrien & Jenny Genesoto 75 Glenbrook (26-3) Mods LS3.70 FD / Reaction Sys / 80mm Front&Intermidiate / Hydroboost / 16" Tires / Frame Rebuild / Interior Rebuild Yuba City,Ca. Text 530-nine-3-three-3-nine-nine-6

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Re: 78 AC vacuum leak [message #312788 is a reply to message #312739]

Fri, 03 February 2017 14:46

thesmith

Messages: 589
Registered: February 2015
Location: Cary, NC

Karma: 6

Senior Member

Got a picture of the wafers? Might be possible to replicate them.

Pete

Cary, NC 1978 Center Kitchen Royale.

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Re: 78 AC vacuum leak [message #312789 is a reply to message #312788]

Fri, 03 February 2017 15:14

A Hamilto

Messages: 4508
Registered: April 2011

Karma: 39

Senior Member

thesmith wrote on Fri, 03 February 2017 14:46

Got a picture of the wafers? Might be possible to replicate them.
Pete

The wafer shown in its housing, on the left:

http://www.gmcmhphotos.com/photos/data/3625/medium/DSC_0630.JPG

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Re: 78 AC vacuum leak [message #312790 is a reply to message #312788]

Fri, 03 February 2017 15:21

C Boyd

Messages: 2629
Registered: April 2006

Karma: 18

Senior Member

There are two rubber wafers. Both are flat on the backs with molded studs that fit in holes in housing to keep from turning.small one is on, off for water valve in heater line. The 9 port one is like a puzzle. I am using washers to face the housing with valve grinding compound.

http://www.gmcmhphotos.com/photos/heater-vacuum-control/p62138-heater-vacuum-control.html

C. Boyd
76 Crestmont
East Tennessee

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