Home » Public Forums » GMCnet » [GMCnet] Disc brake project still in failure mode
Re: [GMCnet] Disc brake project still in failure mode [message #282160 is a reply to message #282157] |
Wed, 15 July 2015 09:23 |
Ken Henderson
Messages: 8726 Registered: March 2004 Location: Americus, GA
Karma:
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Senior Member |
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Just grab the metal push rod and pull it out -- that may require pliers.
If the plastic carrier comes out too, just push it back in after removing
the metal one.
Don't worry; it will all come clear eventually.
Ken H.
On Wed, Jul 15, 2015 at 10:04 AM, Neil Fonville
wrote:
> Now I really realize I'm in over my head. I only understand parts of
> this. So let me work on getting the pin extended. How do I remove the
> metal pin from the black sleeve that is about 1" in diameter that holds the
> metal pin?
>
> Ken: Thanks for taking the time to write this up. I know it's time
> consuming.
>
> Neil
>
> -----Original Message-----
> From: Gmclist [mailto:gmclist-bounces@list.gmcnet.org] On Behalf Of Ken
> Henderson
> Sent: Wednesday, July 15, 2015 8:28 AM
> To: GMCNet
> Subject: Re: [GMCnet] Disc brake project still in failure mode
>
> "...can't be this hard." Sorry, but Yes, it can. When I, with the help
> of highly experienced GMCers Bobby Moore and JR Slaten, installed Eldorado
> rear discs in 2001, we probably pumped 3+ quarts of fluid through the
> system before eliminating all the air. Many have encountered worse. I've
> used "pump & hold", Vacuum at bleeder, Pressure bleeder, and System vacuum
> on various installations. They've all worked, eventually, but not one has
> been easy.
>
> First about that 0.100" pushrod clearance (actually, more like 0.120"):
> Because of the "pedal ratio" (the length of the pedal arm vs the length
> of the bell crank that actually pushes on the booster/master cylinder),
> which is somewhere around 6:1, Add the minimum 0.005" clearance the
> pushrod should have had before you moved the bracket to the 0.120" and you
> have 1/8" pushrod clearance MINIMUM. The result is that the pushrod
> clearance allows the pedal to go down about 3/4" before you even begin to
> try to apply the brakes. And there is some other play in the system, such
> as the input clevis on the bellcrank and in the cross-shaft bushings. 3/4"
> is probably the minimum pedal play you'll see.
>
> Then, before the brakes begin to apply, there's the "slop" in the brakes
> themselves:
>
> 1. The calipers are designed so that the rubber seals retract the pistons
> a tiny bit via hysteresis. That's intended to keep the brakes from
> dragging. I don't think any of us know what that distance is, but remember
> that it's present at 6 locations, and is amplified by the hydraulic
> advantage of the system. I don't know that number off hand but it's on the
> order of 10:1. So, if each caliper moves only 0.005", that's 0.050" at the
> MC, multiplied by that 6:1 pedal ratio, giving another 0.300" -- over 1/4"
> added to your 3/4", or 1" free play.
>
> 2. Then there's disc runout. We'd like the rotor to have NO axial runout
> -- but every one of them does. And on old equipment like our GMC's, it
> can be quite significant, even when one has installed new discs, as you did
> with the Manny Brakes. I've done several of those upgrades and have used a
> dial indicator to measure the runout. It has not proven unusual to see
> 0.010", especially if special care is not taken to clean ALL the rust off
> of the hub where the rotor mounts. If you see more than 0.005" runout on
> any rotor, it needs correction for optimal brake performance. Remember:
> runout causes kick-back of the caliper pistons exactly like in 1.,
> preceding.
>
> 3. Finally, there's the inevitable axial rotor movement due to wheel
> bearing clearance. There shouldn't be much of that at the front wheels,
> but the middle and rear wheels WILL have some. Manual X7525 specifies
> 0.001"-0.005" -- I, and many others prefer the 0.005" or more and it's
> likely that most have at least 0.005". That end play is likewise amplified
> by the system putting the pedal still lower.
>
> My own brake pedal, even with the newly installed Hydroboost, probably
> goes down 1" before achieving any significant braking. I've driven only
> one GMC that did better; it also had Hydroboost and had been worked on very
> intently by Bob Stone and probably had 1/2"+ play. If you can make a
> moderately fast stop with 1/2 pedal, you're probably in pretty good shape.
> But you're going to have to fix that push rod to have any chance of
> getting there.
>
> JMHO,
>
> Ken H.
> Americus, GA
> '76 X-Birchaven w/Cad500/Howell EFI & EBL, Manny Brakes & 1-Ton, etc., etc.
> www.gmcwipersetc.com
>
> On Wed, Jul 15, 2015 at 7:23 AM, Neil Fonville
> wrote:
>
>> I have not lengthen the push rod at this point. I feel the .010 gap
>> when beginning to push the pedal. I guess I’m clueless on the braking
>> system because I don’t understand how .010 gap can lead to 2/3 of the
>> pedal stroke before braking action occurs? Or the pedal being soft?
>>
>> I’m very frustrated at this point and must find some on hands help. I
>> spent about 20 hours installing the kit. 60 trying to bleed it and get
>> the MC replaced. Can’t be this hard.
>>
>> Neil
>>
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Ken Henderson
Americus, GA
www.gmcwipersetc.com
Large Wiring Diagrams
76 X-Birchaven
76 X-Palm Beach
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