80 mm brakes

L

loren \(skip\) newhouse

New member
Aug 27, 1998
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Yesterday, on the way home from a rally, I stopped at a Flying J for
lunch and to dump my holding tank. While doing the latter I noticed that
my brake lights were on. In a minute I noticed that they had gone out so
I didn't think anymore about it. After I got on the highway I had some
difficulty getting my cruise control to work. About 15 miles farther
down the road we started to smell something hot so I pulled into a mall
parking lot for a check Before stopping, I noticed that the temp gauge
was getting close to the halfway mark . During my walk around I noticed
that the brake lights were lighted again. I got in the driver's seat and
asked my co-pilot to tell me if and when the lights went out. I stepped
on the pedal sharply. Lights stayed on. I put my foot under the pedal
and pulled up. The lights went out. I then checked the front wheels for
heat and found that they were quite warm and I could smell the same odor
that had entered the coach. We left the lot and cautiously drove the 20
more miles to home.

My suspicions are that a front brake hose has deteriorated preventing
the fluid from caliper cylinder to return to the master cylinder. Does
this sound logical?

Using the above assumption, I decided that this would be the right time
to upgrade to 80 mm calipers. However, I didn't know if a PO had already
done so. I pulled the right front wheel and loosened the caliper from
the rotor. I took the existing pads out and headed off to AutoZone with
my NET parts list. At the store I compared the size or the PF 1534 pads
with my old pads and found them to be very close in size(no appreciable
difference.) I went back to the coach and measured the piston diameter
of my old caliper (~72mm) and the piston diameter of the AZ 90184
(~75mm.) Does It sound like I already have larger than standard
calipers? I went back to AZ to measure a replacement caliper for the
original. They didn't have one in stock. Neither did NAPA.
NAPA did tell me that their part numbers 42-2010 & 42-2011 are for Cad
& Olds 1966-1978.
On my existing brakes I do not have a banjo hose so if I stay with my
calipers I'll need to search out a replacement number(s). Any guidance,
comments, and/or criticism will be appreciated.
Skip Newhouse
'75 Avion with hot stinky brakes
Western MD
 
Skip

Sorry you had the scare. Call LEIGH hARRISON, 703-494-9914. He probably
has forgotten more about GMC brakes than most will ever know. If he can't
help, call Norman Dubner (look in the chapter membership list) I talked to
him this afternoon about eventually upgrading.

Wayne Newland (husband of balloon flyer)

> Yesterday, on the way home from a rally, I stopped at a Flying J for
> lunch and to dump my holding tank. While doing the latter I noticed that
> my brake lights were on. In a minute I noticed that they had gone out so
> I didn't think anymore about it. After I got on the highway I had some
> difficulty getting my cruise control to work. About 15 miles farther
> down the road we started to smell something hot so I pulled into a mall
> parking lot for a check Before stopping, I noticed that the temp gauge
> was getting close to the halfway mark . During my walk around I noticed
> that the brake lights were lighted again. I got in the driver's seat and
> asked my co-pilot to tell me if and when the lights went out. I stepped
> on the pedal sharply. Lights stayed on. I put my foot under the pedal
> and pulled up. The lights went out. I then checked the front wheels for
> heat and found that they were quite warm and I could smell the same odor
> that had entered the coach. We left the lot and cautiously drove the 20
> more miles to home.
>
> My suspicions are that a front brake hose has deteriorated preventing
> the fluid from caliper cylinder to return to the master cylinder. Does
> this sound logical?
>
> Using the above assumption, I decided that this would be the right time
> to upgrade to 80 mm calipers. However, I didn't know if a PO had already
> done so. I pulled the right front wheel and loosened the caliper from
> the rotor. I took the existing pads out and headed off to AutoZone with
> my NET parts list. At the store I compared the size or the PF 1534 pads
> with my old pads and found them to be very close in size(no appreciable
> difference.) I went back to the coach and measured the piston diameter
> of my old caliper (~72mm) and the piston diameter of the AZ 90184
> (~75mm.) Does It sound like I already have larger than standard
> calipers? I went back to AZ to measure a replacement caliper for the
> original. They didn't have one in stock. Neither did NAPA.
> NAPA did tell me that their part numbers 42-2010 & 42-2011 are for Cad
> & Olds 1966-1978.
> On my existing brakes I do not have a banjo hose so if I stay with my
> calipers I'll need to search out a replacement number(s). Any guidance,
> comments, and/or criticism will be appreciated.
> Skip Newhouse
> '75 Avion with hot stinky brakes
> Western MD
 
> -- [ From: Eugene Fisher * EMC.Ver #3.1a PRODIGY ] --
>
> Could be you have some other problem
>
> sticky brake pedal
> vacuum booster not returning
> Master cyl not returning
>
> My guess is that it might be the vacuum booster. If it is not releasing
> it would hold on the brakes.
>
> The brake light you talk about, is that the brake lights on the rear of
> the coach?
>
> gene
>
> Gene 76Palm Beach /Or/CA
>
> GMC Technical Information
> http://www.california.com/~eagle/

Hi Gene,
You'll probably have to talk me thru this, but:
1. I sharply stepped on the brake pedal to free it up if it was sticking.
Couldn't detect any sticking. I'll look at it closer
tomorrow and lube where it might be sticking.
2. The vacuum booster might not be returning. How do I check? I recently
installed an auxiliary vacuum pump using the instructions I found on your
website. The pump works fine. Not too often, but usually in stop & go
traffic in town. Thanx for that
3. How do I check if the master cylinder is not returning?
4. The brake lights I spoke of are at the rear of the coach.

Do you have any information on the caliper size question?

Thanx for responding.
Skip
'75 Avion
Western MD
 
>
>My suspicions are that a front brake hose has deteriorated preventing
>the fluid from caliper cylinder to return to the master cylinder. Does
>this sound logical?
>
>

Quite common failure, the inner wall of the hose will fail, the brake
fluid will enter between the liner/wire web and prevent the fluid from
returning. We had that happen on our coach, after I determined what the
problem was and replaced the hoses (both sides) I cut the hose apart to
find the reason.. VERY LARGE bubble in the inner hose wall..

John & Brenda Szalay
73 PD
 
I had a similar problem on a '79 GMC mini chassis. The right front brake
would drag. This was caused by a hose clamp pinching the line. My cousin
who runs a brake shop crawled under and opened up the clamp a little and it
then worked fine. He said it was a common problem in GMC's of that era.

FWIW

Dick

> Quite common failure, the inner wall of the hose will fail, the brake
>fluid will enter between the liner/wire web and prevent the fluid from
>returning. We had that happen on our coach, after I determined what the
>problem was and replaced the hoses (both sides) I cut the hose apart to
>find the reason.. VERY LARGE bubble in the inner hose wall..
>
> John & Brenda Szalay
> 73 PD
>
>
>
 
>
> Using the above assumption, I decided that this would be the
> right time to upgrade to 80 mm calipers. However, I didn't
> know if a PO had already done so.

> On my existing brakes I do not have a banjo hose so if I stay with my
> calipers I'll need to search out a replacement number(s). Any
> guidance, comments, and/or criticism will be appreciated.

Skip,

If your caliper has a screw-in hose, then it's probably the original. The
80mm calipers use a banjo fitting. IIRC, Autozone stocks a replacement hose
for the original caliper - don't have the number in front of me right now
tho'.

Patrick
 
> There were many many postings in Jan. on this whole subject. You might want
> to check the archives to look at some of the responses to the retrofit.
>
> The Autozone caliper numbers are: 90184 and 90185 for $12.99 each plus core.
>
> the hoses are 3751339 77320 and 3751347 77421 at $24.99 each.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM

Emery,

Thanks for the advice. But, my concern is not knowing if a PO had already done
the 80 mm upgrade. The existing pad is similar in size to the 80 mm pad (PF
1534) and the existing piston is very close in size to the piston size on the
replacement caliper (~72mm vs ~75mm.) I don't know just how the 80 mm dimension
is determined. A previous response said that if my hoses are not banjo type
then I probably have the original calipers. I was hoping that someone who has an
original caliper could/would measure across the piston and tell me what they
found. It doesn't seem to me that the differences from my old to the new would
make an appreciable difference in performance.
As suggested earlier, I checked and lubed my brake pedal pivot. Then I tried to
put on the brakes so the brake light would stay on so I could check fluid
pressure at the bleed valve. Couldn't get it to happen. I can't see and bulging
of the brake lines. Any more comments will be appreciated.
Skip
'75 Avion in Western MD
 
Skip:

What Emery told you previously about the difference in hose attachment
between the original and the 80 mm (i. e. banjo type fitting will tell you
which caliper you have. I don't believe there is any difference in the
brake pad size between the two so that won't help you. If there is doubt
about the piston size, why can't you go to AutoZone and have them measure
the P/N that Emery gave you. In fact, why can't you phone them to do it and
save your transportation.

An 80 mm caliper is supposed to give you ~ 1000 psi more braking force vs.
the original caliper.

Paul Bartz

From: Loren (Skip) Newhouse [mailto:lnewh]
Sent: Tuesday, August 24, 1999 2:51 PM
Subject: Re: GMC: 80 mm brakes

> There were many many postings in Jan. on this whole subject. You
might want
> to check the archives to look at some of the responses to the
retrofit.
>
> The Autozone caliper numbers are: 90184 and 90185 for $12.99 each
plus core.
>
> the hoses are 3751339 77320 and 3751347 77421 at $24.99 each.
>
> Emery Stora
> 77 Kingsley
> Santa Fe, NM

Emery,
Thanks for the advice. But, my concern is not knowing if a PO had already
done the 80 mm upgrade. The existing pad is similar in size to the 80 mm pad
(PF 1534) and the existing piston is very close in size to the piston size
on the replacement caliper (~72mm vs ~75mm.) I don't know just how the 80
mm dimension is determined. A previous response said that if my hoses are
not banjo type then I probably have the original calipers. I was hoping that
someone who has an original caliper could/would measure across the piston
and tell me what they found. It doesn't seem to me that the differences from
my old to the new would make an appreciable difference in performance. As
suggested earlier, I checked and lubed my brake pedal pivot. Then I tried to
put on the brakes so the brake light would stay on so I could check fluid
pressure at the bleed valve. Couldn't get it to happen. I can't see and
bulging of the brake lines. Any more comments will be appreciated.
Skip
'75 Avion in Western MD
 
> I can't see and bulging of the brake lines.

Skip,

The failure will be internal and can only be seen by cutting open the old
line as John Szalay wrote earlier today.

Patrick
 
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