Home » Public Forums » GMCnet » [GMCnet] Disc brake project still in failure mode
| Re: [GMCnet] Disc brake project still in failure mode [message #282153 is a reply to message #282148] |
Wed, 15 July 2015 08:28   |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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Senior Member |
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"...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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