1-1/8" rear brake cylinders
- Thread starter dave1
- Start date
Dave:
Are you a member of GMCMI??? If not, recommend you join and get their
publications. Parts list alone, is well worth the membership price.
Their Parts Interchange Index gives: Bendix 29634 and CarQuest EW86000.
Paul Bartz
From: Dave [mailto
GMDGM]
Sent: Monday, June 21, 1999 5:27 PM
Subject: GMC: 1-1/8" rear brake cylinders
Does anyone on the list have a GM part number or an application for the
1-1/8" rear brake cylinders? If so, would you please post it.
Are you a member of GMCMI??? If not, recommend you join and get their
publications. Parts list alone, is well worth the membership price.
Their Parts Interchange Index gives: Bendix 29634 and CarQuest EW86000.
Paul Bartz
From: Dave [mailto
GMDGM]Sent: Monday, June 21, 1999 5:27 PM
Subject: GMC: 1-1/8" rear brake cylinders
Does anyone on the list have a GM part number or an application for the
1-1/8" rear brake cylinders? If so, would you please post it.
I did the upgrade to 1-1/8" brake cylinders on all four rear wheels last
week. I decided to use the same size cylinders on all four rear wheels. I
made the decision for two reasons. First, this is the way GM decided to do
it (same size on all four). Second, from what I understand, this is how the
load is distributed under normal braking conditions (same load on all four
rears).
I used the CarQuest EW86000 parts. The biggest problem I had was trying to
get four brake cylinders that where all the same. It looks like carquest
stocks various manufacturers parts all under the same CarQuest part number.
Fortunately the local carquest store was helpful in finding four cylinders
from the same manufacturer.
While rummaging through their stock I managed to get a couple more numbers
for the 1-1/8 cylinders. All of the following numbers where stocked under
the CarQuest EW86000 stock number:
Mfg P/N
----------------------
Bendix 29634
Wagner 108361
? K27-747
The upgrade was easy. The 1-1/8 cylinders are a direct swap. I did not
allow air into the system so I had no problems with bleeding it. After
removing the brake line from an old cylinder I put a small rubber cap over
the line and replaced the lost fluid in the master cylinder. After
installing the new cylinder and allowing gravity to purge the air out of
the new cylinder I once again refilled the master cylinder. Using this
technique for each cylinder I was able to flush the system and replace all
four cylinders without introducing any air into the system.
I am very impressed with the resulting improvement in braking. The peddle
stroke is only slightly longer and I no longer feel like I need to stand on
the brakes during a normal stop. I may still do the 80mm upgrade to the
fronts but I am thinking that I will drive it as-is for a while before I
make that decision.
I tried doing some skid tests after upgrading the rears to see if the rear
wheels lock up prematurely. One thing I learned is that it is very
difficult to lock up any of the wheels. Even on gravel roads or wet
pavement. At this point I would say that the skid test results are
inconclusive. I can say that upgrading to 1-1/8" cylinders does not seem to
make the rear brakes overly sensitive. But, during normal stopping, the
peddle pressure feels a lot more reasonable.
Makes me wonder what, if any, braking tests GM did during the design of the
GMC motorhome. The larger rear cylinders make a big difference but it seems
like there is still a lot of room for improvement. I would be interested in
knowing what, if any, reasons GM had for not using the 1-1/8 cylinders for
the original brake design.
I wonder if any of the engineers who worked on the original GMC design are
on-line. It sure would be interesting to hear their thoughts on why things
where done the way they where...
Dave
73 Sequoia
week. I decided to use the same size cylinders on all four rear wheels. I
made the decision for two reasons. First, this is the way GM decided to do
it (same size on all four). Second, from what I understand, this is how the
load is distributed under normal braking conditions (same load on all four
rears).
I used the CarQuest EW86000 parts. The biggest problem I had was trying to
get four brake cylinders that where all the same. It looks like carquest
stocks various manufacturers parts all under the same CarQuest part number.
Fortunately the local carquest store was helpful in finding four cylinders
from the same manufacturer.
While rummaging through their stock I managed to get a couple more numbers
for the 1-1/8 cylinders. All of the following numbers where stocked under
the CarQuest EW86000 stock number:
Mfg P/N
----------------------
Bendix 29634
Wagner 108361
? K27-747
The upgrade was easy. The 1-1/8 cylinders are a direct swap. I did not
allow air into the system so I had no problems with bleeding it. After
removing the brake line from an old cylinder I put a small rubber cap over
the line and replaced the lost fluid in the master cylinder. After
installing the new cylinder and allowing gravity to purge the air out of
the new cylinder I once again refilled the master cylinder. Using this
technique for each cylinder I was able to flush the system and replace all
four cylinders without introducing any air into the system.
I am very impressed with the resulting improvement in braking. The peddle
stroke is only slightly longer and I no longer feel like I need to stand on
the brakes during a normal stop. I may still do the 80mm upgrade to the
fronts but I am thinking that I will drive it as-is for a while before I
make that decision.
I tried doing some skid tests after upgrading the rears to see if the rear
wheels lock up prematurely. One thing I learned is that it is very
difficult to lock up any of the wheels. Even on gravel roads or wet
pavement. At this point I would say that the skid test results are
inconclusive. I can say that upgrading to 1-1/8" cylinders does not seem to
make the rear brakes overly sensitive. But, during normal stopping, the
peddle pressure feels a lot more reasonable.
Makes me wonder what, if any, braking tests GM did during the design of the
GMC motorhome. The larger rear cylinders make a big difference but it seems
like there is still a lot of room for improvement. I would be interested in
knowing what, if any, reasons GM had for not using the 1-1/8 cylinders for
the original brake design.
I wonder if any of the engineers who worked on the original GMC design are
on-line. It sure would be interesting to hear their thoughts on why things
where done the way they where...
Dave
73 Sequoia
Dave where did you get the idea from that GM intended to use the same size
rear wheel cylinders on all wheels. It is not true. The rears have always
had smaller cylinders than the intermediate ones. In addition you have made
a false assumption that the same load is on each rear wheel. It is not. In
fact no two wheels on the GMC motorhome will likely have the same loading at
the same time. In a real panic stop the rear wheels wil lock up and skid
before the intermediate wheels reach full braking. the fact is in a full
stop weight transfers from the rear axels to the intermediate ones.
>I did the upgrade to 1-1/8" brake cylinders on all four rear wheels last
>week. I decided to use the same size cylinders on all four rear wheels. I
>made the decision for two reasons. First, this is the way GM decided to do
>it (same size on all four). Second, from what I understand, this is how the
>load is distributed under normal braking conditions (same load on all four
>rears).
>
>I used the CarQuest EW86000 parts. The biggest problem I had was trying to
>get four brake cylinders that where all the same. It looks like carquest
>stocks various manufacturers parts all under the same CarQuest part number.
>Fortunately the local carquest store was helpful in finding four cylinders
>from the same manufacturer.
>
>While rummaging through their stock I managed to get a couple more numbers
>for the 1-1/8 cylinders. All of the following numbers where stocked under
>the CarQuest EW86000 stock number:
>
> Mfg P/N
> ----------------------
> Bendix 29634
> Wagner 108361
> ? K27-747
>
>The upgrade was easy. The 1-1/8 cylinders are a direct swap. I did not
>allow air into the system so I had no problems with bleeding it. After
>removing the brake line from an old cylinder I put a small rubber cap over
>the line and replaced the lost fluid in the master cylinder. After
>installing the new cylinder and allowing gravity to purge the air out of
>the new cylinder I once again refilled the master cylinder. Using this
>technique for each cylinder I was able to flush the system and replace all
>four cylinders without introducing any air into the system.
>
>I am very impressed with the resulting improvement in braking. The peddle
>stroke is only slightly longer and I no longer feel like I need to stand on
>the brakes during a normal stop. I may still do the 80mm upgrade to the
>fronts but I am thinking that I will drive it as-is for a while before I
>make that decision.
>
>I tried doing some skid tests after upgrading the rears to see if the rear
>wheels lock up prematurely. One thing I learned is that it is very
>difficult to lock up any of the wheels. Even on gravel roads or wet
>pavement. At this point I would say that the skid test results are
>inconclusive. I can say that upgrading to 1-1/8" cylinders does not seem to
>make the rear brakes overly sensitive. But, during normal stopping, the
>peddle pressure feels a lot more reasonable.
>
>Makes me wonder what, if any, braking tests GM did during the design of the
>GMC motorhome. The larger rear cylinders make a big difference but it seems
>like there is still a lot of room for improvement. I would be interested in
>knowing what, if any, reasons GM had for not using the 1-1/8 cylinders for
>the original brake design.
>
>I wonder if any of the engineers who worked on the original GMC design are
>on-line. It sure would be interesting to hear their thoughts on why things
>where done the way they where...
>
>Dave
>73 Sequoia
>
>
>
rear wheel cylinders on all wheels. It is not true. The rears have always
had smaller cylinders than the intermediate ones. In addition you have made
a false assumption that the same load is on each rear wheel. It is not. In
fact no two wheels on the GMC motorhome will likely have the same loading at
the same time. In a real panic stop the rear wheels wil lock up and skid
before the intermediate wheels reach full braking. the fact is in a full
stop weight transfers from the rear axels to the intermediate ones.
>I did the upgrade to 1-1/8" brake cylinders on all four rear wheels last
>week. I decided to use the same size cylinders on all four rear wheels. I
>made the decision for two reasons. First, this is the way GM decided to do
>it (same size on all four). Second, from what I understand, this is how the
>load is distributed under normal braking conditions (same load on all four
>rears).
>
>I used the CarQuest EW86000 parts. The biggest problem I had was trying to
>get four brake cylinders that where all the same. It looks like carquest
>stocks various manufacturers parts all under the same CarQuest part number.
>Fortunately the local carquest store was helpful in finding four cylinders
>from the same manufacturer.
>
>While rummaging through their stock I managed to get a couple more numbers
>for the 1-1/8 cylinders. All of the following numbers where stocked under
>the CarQuest EW86000 stock number:
>
> Mfg P/N
> ----------------------
> Bendix 29634
> Wagner 108361
> ? K27-747
>
>The upgrade was easy. The 1-1/8 cylinders are a direct swap. I did not
>allow air into the system so I had no problems with bleeding it. After
>removing the brake line from an old cylinder I put a small rubber cap over
>the line and replaced the lost fluid in the master cylinder. After
>installing the new cylinder and allowing gravity to purge the air out of
>the new cylinder I once again refilled the master cylinder. Using this
>technique for each cylinder I was able to flush the system and replace all
>four cylinders without introducing any air into the system.
>
>I am very impressed with the resulting improvement in braking. The peddle
>stroke is only slightly longer and I no longer feel like I need to stand on
>the brakes during a normal stop. I may still do the 80mm upgrade to the
>fronts but I am thinking that I will drive it as-is for a while before I
>make that decision.
>
>I tried doing some skid tests after upgrading the rears to see if the rear
>wheels lock up prematurely. One thing I learned is that it is very
>difficult to lock up any of the wheels. Even on gravel roads or wet
>pavement. At this point I would say that the skid test results are
>inconclusive. I can say that upgrading to 1-1/8" cylinders does not seem to
>make the rear brakes overly sensitive. But, during normal stopping, the
>peddle pressure feels a lot more reasonable.
>
>Makes me wonder what, if any, braking tests GM did during the design of the
>GMC motorhome. The larger rear cylinders make a big difference but it seems
>like there is still a lot of room for improvement. I would be interested in
>knowing what, if any, reasons GM had for not using the 1-1/8 cylinders for
>the original brake design.
>
>I wonder if any of the engineers who worked on the original GMC design are
>on-line. It sure would be interesting to hear their thoughts on why things
>where done the way they where...
>
>Dave
>73 Sequoia
>
>
>
>Dave where did you get the idea from that GM intended to use the same size
>rear wheel cylinders on all wheels.
The parts book and from looking at the cylinders that I removed from my
four rears.
Maybe the design changed at a later date. I have a 73, updated per the
recall in 88.
>It is not true. The rears have always
>had smaller cylinders than the intermediate ones.
Are you sure??
>In addition you have made
>a false assumption that the same load is on each rear wheel.
I think you misunderstood me. What I was saying is that the braking load is
the same on all four rears during normal braking. I agree that, in a panic
stop there will be more load on the mids than the rears. But, unless one of
the tires begins skidding, the braking load (a function of wheel speed and
pressure) will be the same.
Dave
73 Sequoia
>rear wheel cylinders on all wheels.
The parts book and from looking at the cylinders that I removed from my
four rears.
Maybe the design changed at a later date. I have a 73, updated per the
recall in 88.
>It is not true. The rears have always
>had smaller cylinders than the intermediate ones.
Are you sure??
>In addition you have made
>a false assumption that the same load is on each rear wheel.
I think you misunderstood me. What I was saying is that the braking load is
the same on all four rears during normal braking. I agree that, in a panic
stop there will be more load on the mids than the rears. But, unless one of
the tires begins skidding, the braking load (a function of wheel speed and
pressure) will be the same.
Dave
73 Sequoia
