Brake bleeding & other myths

[thomas g. warner]

If you have a 1975 or 1976 model, and have the original brakes, master
cylinder and combination valve, the correct brake bleeding sequence is as I
noted in my previous post. See 1975 and 1976 Maintenance Manual Motorhome
ZEO 6581, page 5-14, figure 1. I am taking this information from an
origninal GMC manual not a reprint.

I also noticed that early someone commented that the manual was also
incorrect relative to the direction of distributer rotation for the 455, and
that Dick Paterson had noted such in a seminar. Dick Paterson is a very
informed individual and I respect his opinion, however and I am sure that he
knows that the distributer rotates counter clockwise. Someone therefore
must have misunderstood him. Could this be how myths are started?

In the above manual page 6Y-34, figure 32, the Oldsmobile 455 engine
distributer is shown as rotating counter clockwise.

The problem people are having bleeding brakes may stem from the various
modifications that individuals are putting on their coaches. Rear disc
brakes complicates the bleeding procedure.



>On 4/29, Tom Warner writes;
>"Proper sequence for bleeding the brakes is:
>
>1. Left front wheel
>2. Right front wheel
>3. left front bogie
>4. left rear bogie
>5. right front bogie
>6. Right rear bogie"
>
>HELP!!
>
>The manual (X-7625) for my '76 says exactly the opposite!
>
>I'm trying to provide tips to an otherwise very experienced mechanic as to
>what works on a GMC, but this is very confusing. I know that there are
>varying opinions on the preferred methods for accomplishing certain tasks,
>but opposite techniques seem guite a bit more controversial.
>
>Does it depend on which method is being used (push and hold the pedal,
>vacuum bleeder, or pressure bleeder)?
>
>I remember many months back that Heinz was having lots of trouble getting
>his system bled, and finally arrived at a sequence that worked for him.
>Which was it Heinz; front to back, or back to front. I note that Scott got
>good results using the back to front sequence called for in the manual.
>
>My brakes are being done next week, and if there really is a "right" way to
>bleed the system, it would sure be helpful to know which it is, that is if
>it really makes a difference.
>
>I suspect many others may like to know if there is a sequence which works best.
>
>Or is my feeble mind missing something here?
>
>Thanks to any who offer opinions,
>
> ___________
>Dave (& Dege), '76 Royale /_][__] [_] | "SR JAMES"
> Santa Barbara, CA *-0------OO--* (our hobby)
>
>
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
"The beautiful Mohawk Vally"

 

[bartz paul]

Thomas:

I'm the one who addressed the distributor rotation. In the 77-78
Supplement, pages 6Y-33 and 34 states that rotation is clockwise, which is
what I said. However, Dick P pointed out that it is in fact
counterclockwise, again what I said. No myth Thomas, that's a fact.

Paul Bartz

From: Thomas G. Warner
Sent: 4/30/99 6:13 PM
Subject: Re: GMC: Brake bleeding & other myths

If you have a 1975 or 1976 model, and have the original brakes, master
cylinder and combination valve, the correct brake bleeding sequence is
as I
noted in my previous post. See 1975 and 1976 Maintenance Manual
Motorhome
ZEO 6581, page 5-14, figure 1. I am taking this information from an
origninal GMC manual not a reprint.

I also noticed that early someone commented that the manual was also
incorrect relative to the direction of distributer rotation for the 455,
and
that Dick Paterson had noted such in a seminar. Dick Paterson is a very
informed individual and I respect his opinion, however and I am sure
that he
knows that the distributer rotates counter clockwise. Someone therefore
must have misunderstood him. Could this be how myths are started?

In the above manual page 6Y-34, figure 32, the Oldsmobile 455 engine
distributer is shown as rotating counter clockwise.

The problem people are having bleeding brakes may stem from the various
modifications that individuals are putting on their coaches. Rear disc
brakes complicates the bleeding procedure.



>On 4/29, Tom Warner writes;
>"Proper sequence for bleeding the brakes is:
>
>1. Left front wheel
>2. Right front wheel
>3. left front bogie
>4. left rear bogie
>5. right front bogie
>6. Right rear bogie"
>
>HELP!!
>
>The manual (X-7625) for my '76 says exactly the opposite!
>
>I'm trying to provide tips to an otherwise very experienced mechanic as
to
>what works on a GMC, but this is very confusing. I know that there are
>varying opinions on the preferred methods for accomplishing certain
tasks,
>but opposite techniques seem guite a bit more controversial.
>
>Does it depend on which method is being used (push and hold the pedal,
>vacuum bleeder, or pressure bleeder)?
>
>I remember many months back that Heinz was having lots of trouble
getting
>his system bled, and finally arrived at a sequence that worked for him.
>Which was it Heinz; front to back, or back to front. I note that Scott
got
>good results using the back to front sequence called for in the manual.
>
>My brakes are being done next week, and if there really is a "right"
way to
>bleed the system, it would sure be helpful to know which it is, that is
if
>it really makes a difference.
>
>I suspect many others may like to know if there is a sequence which
works best.
>
>Or is my feeble mind missing something here?
>
>Thanks to any who offer opinions,
>
> ___________
>Dave (& Dege), '76 Royale /_][__] [_] | "SR JAMES"
> Santa Barbara, CA *-0------OO--* (our hobby)
>
>
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
"The beautiful Mohawk Vally"

 

[dave lowry]

We're getting closer...........

I have a Cinnabar reprint (X-7525A) of the manual you cite, and it also
calls out the front to back sequence. However, I also have the supplement
(X-7625) which states in it's foreward as applicable to "all 1976
motorhomes and transmodes" (which is what I have). It states "A new brake
bleeding sequence is shown in figure 1. This new sequence reduces the
possibility of having to rebleed wheel cylinders to eliminate air from the
system."

Sorry Thomas, but I can't help wondering why GM would make such a statement
if there were'nt some good reason. Can someone with the supplement for the
1977 & 1978 models (X-7725 ) tell us what it has to say on this subject?
Could it be that GM changed their recommendation, for some reason,
effective with the 1976 models? And if so, what was their logic?

With my very limited knowledge of brake systems, I know of only three ways
to reintroduce air into a circuit which has already been bled; 1) letting
the master cylinder or other source run out of fluid during the process,
thus injecting air instead of fluid, 2) not properly sealing the already
bled cylinders, thus allowing them to suck air back in as subsequent
cylinders are bled, or 3) allowing the combination valve to suck air from
an unbled circuit into one being bled. Since the first two are relatively
easy to avoid, I suspect the majority of problems being experienced can be
traced to not securing the combination valve as specified in the manual.
And BTW, what's the best alternative to the special GM tool #J23709.

Sorry to keep asking so many questions, but this is one area where there
seems to be plenty of confusion and which is absolutely critical to all of
our safety.

Regards,

___________
Dave (& Dege), '76 Royale /_][__] [_] | "SR JAMES"
Santa Barbara, CA *-0------OO--* (our hobby)

 

[thomas g. warner]

emery it does not make sense to me to bleed the farthest brake first unless
you are forcing brake fluid INTO that line towards the master cylinder, ie;
a pressure bleed system.

However if you are using the master cylinder to pump up the brakes and
force the air out( like is normally done), it would seem defeating to bleed
the farthest brake first.If you did as soon as you tried to bleed the next
brake, air could enter the line already bled. Does that make sense or not?

>

>
>1977 & 1978 models (X-7725 ) tell us what it has to say on this subject? >>
>
>The X-7725 basically says that GM recommends changing the position of
>bleeding from the farthest wheel to the closest wheel.
>This also applies to older models as they didn't change the brake system. GM
>says that the change will help prevent having to bleed twice. If you ask any
>brake shop I'm sure that you'll find that the mechanics always expect to
>bleed the farthest wheel first.
>
>Emery Stora
>77 Kingsley
>Santa Fe, NM
>
>
Tom & Marg Warner
Vernon Center NY
1976 palmbeach
"The beautiful Mohawk Vally"

 

[ron & gina]

Just had to put my two cents in. I'm no master mechanic, but mechanically
incline I'd like to think myself as and for my fifty years of messing with
cars I was ALWAYS told to bleed the furthest wheel first and work your way
towards the master cylinder. And on all my cars/trucks this has worked fine
on the first try. Just my two cents!

Ron&Gina
73 Pumpkin-Rose
Fort Mohave, Az.

- -----Original Message-----
From: Tom Warner
To: gmcmotorhome
Date: Saturday, May 01, 1999 7:32 PM
Subject: Re: GMC: Brake bleeding & other myths

>emery it does not make sense to me to bleed the farthest brake first unless
>you are forcing brake fluid INTO that line towards the master cylinder, ie;
>a pressure bleed system.
>
>However if you are using the master cylinder to pump up the brakes and
>force the air out( like is normally done), it would seem defeating to bleed
>the farthest brake first.If you did as soon as you tried to bleed the next
>brake, air could enter the line already bled. Does that make sense or not?
>
>
>
>
>
>
>

>>

>>
>>>1977 & 1978 models (X-7725 ) tell us what it has to say on this subject?
>>
>>
>>The X-7725 basically says that GM recommends changing the position of
>>bleeding from the farthest wheel to the closest wheel.
>>This also applies to older models as they didn't change the brake system.
GM
>>says that the change will help prevent having to bleed twice. If you ask
any
>>brake shop I'm sure that you'll find that the mechanics always expect to
>>bleed the farthest wheel first.
>>
>>Emery Stora
>>77 Kingsley
>>Santa Fe, NM
>>
>>
>Tom & Marg Warner
>Vernon Center NY
>1976 palmbeach
>"The beautiful Mohawk Vally"
>