78 AC vacuum leak

C

charles boyd

New member
Jul 8, 2007
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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
 
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

>
> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
> ...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
--
73 23' Sequoia 4 Sale
73 23' CanyonLands Parts Unit 4 Sale
Upper Alabama
"When I grow up I am going to be a curmudgeon."
 
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
Source America First
 
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
 
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

>
> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
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.
 
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
 
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

>
> 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
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
 
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
Source America First
 
Charles, PM sent.

--
”When we avoid the mistakes we might have made, we sometimes make the mistakes that we might have avoided.”

Adrien & Jenny Genesoto
75 Glenbrook 26-3
Yuba City,Ca. Text 530-nine-3-three-3-nine-nine-6
 
Got a picture of the wafers? Might be possible to replicate them.

Pete
--
Cary, NC

No Coach yet but likely center kitchen, dry side bath Royale, 78, 403, hopefully with good original white Imron paint.
 
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