Home » Public Forums » GMCnet » [GMCnet] Disc brake project still in failure mode
| Re: [GMCnet] Disc brake project still in failure mode [message #281984 is a reply to message #281981] |
Mon, 13 July 2015 07:13   |
Ken Henderson
Messages: 8726
Registered: March 2004
Location: Americus, GA
Karma:
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Senior Member |
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Neil,
In the recess of the booster, where the MC seats, around the plastic
carrier for the MC push rod, there should be a rubber seal with radial
indentations. That seal is there to retain the vacuum in the forward
chamber of the booster. When assembled normally (with the bracket on the
forward side of the MC ears), the nose of the MC keeps that seal in
position. Now that you've moved the MC 0.100" or so away from the booster,
that seal is no longer retained and MAY be losing vacuum. Dave Lenzi makes
a little nylon washer to fit between the MC and the seal restore the
retaining force; I dont' know whether he sells them separately from his
booster, but you could easily make something similar.
Further compounding the MC/Booster interface is the fact that many of the
boosters being promoted for us today are metrically dimensioned. That
results in the MC seating less snugly in the booster than it should. Dave
considers that a significant problem and makes a centering ring. I'm not
as sure it's a problem because the assemblies I've seen (far fewer than
Dave has) are well centered by the mounting bolts. But you should be aware
of it.
Did you adjust the length of your MC push rod to compensate for the 0.100"
"spacer" the bracket now forms between the MC and the booster? If not, you
will definitely have a low pedal, regardless of the "tightness" of the rest
of the system. Here's how I measure the new push rod:
http://www.gmcmhphotos.com/photos/g6289-master-cylinder-pushrod-gauge.html
HTH,
Ken H.
On Mon, Jul 13, 2015 at 7:56 AM, Neil Fonville wrote:
> Ken, I have a few questions:
>
> I did the procedure of bending the support bracket so I have the pushrod
> gap the thickness of the bracket in pedal play. That does not bother me
> as it¹s so slight. But you discuss a vacuum seal which I don¹t really
> understand. The vacuum itself stays in the booster right? There is no
> vacuum connection to the MC? Booster seems to be working as the pedal is
> much harder when the engine is not running. What am I not understanding
> about the booster to MC connection?
>
> I¹m glad you mention the pressure gauge. I purchased that but have not
> used it yet but will. Do you have to bleed for it or does it self bleed
> and remove the air?
>
> I will closely inspect the front hoses.
>
> Thanks,
> Neil
>
>
>
> On 7/12/15, 8:01 PM, "Gmclist on behalf of Ken Henderson"
>
> wrote:
>
>> Neil,
>>
>> There are so MANY possibilities that I don't know where to start (and I'm
>> in the midst of a Hydroboost installation on my 1-Ton+Manny Brakes
>> equipped
>> 23', which has had just about every brake combination you can think of.
>>
>> I'm just going to mention some items to be sure you've addressed all of
>> them:
>>
>> Booster -- what's its' status?
>>
>> Vacuum seal between the booster and MC -- correct seal, correctly
>> installed?
>>
>> Push rod length -- have you measured it and set about 0.005" clearance?
>>
>> Flex hoses (on front, obviously you installed new rear with Manny Brakes).
>>
>> How much have you driven the coach to break in the pads? It an be amazing
>> how much improvement occurs in the first 100-200 miles.
>>
>> Perhaps the most useful tool for you would be one of these:
>>
>>
> http://www.amazon.com/SSBC-A1704-Brake-Pressure-Gauge/dp/B003VYVFSS/ref=sr
>> _1_1?ie=UTF8&qid=1436748655&sr=8-1&keywords=brake+pressure+gauge
>>
>> That kit from Stainless Steel Brakes includes a set of adapters for 'most
>> any caliper or wheel cylinder to allow you to attach the included 0-1600
>> psi gauge. With that you can measure the hydraulic line pressure at a
>> front and a rear caliper. Unless your booster/MC/etc. can provide 600+
>> psi
>> of line pressure, you will NOT have good brakes. Having a visual
>> indication of the results of each action you take will be valuable in
>> solving your problem. You'd like to have 1000+ psi, but it's rare on a
>> GMC.
>>
>> HTH,
>>
>> Ken H.
>>
>>
>> On Sun, Jul 12, 2015 at 8:19 PM, Neil Fonville
>> wrote:
>>
>>> I'm back to working on my brake project and just not getting it done.
>>> Still have soft pedal and it's takes way too much distance to stop.
>>> It's
>>> far worse from where is started with 2 Disc and 4 Drums that were not
>>> working good.
>>>
>>> Here is rehash of what steps I have taken and maybe someone can help get
>>> me back on track. I'm still amazed I can't find a mechanic/shop in the
>>> Dallas area that could help me with my projects.
>>>
>>> First I installed the Manny rear disc brake kit. Install was straight
>>> forward.
>>> Bleed the system with the standard 2 man pump and release fluid method.
>>> Never felt right and the vehicle was not even remotely drivable.
>>> Replaced proportion valve with Disc/Disc model.
>>> Removed OEM MC and replaced MC with new one from O'rilly. Used the P30
>>> (PN
>>> NMC1668).
>>> Did not bench bleed. Bleed with 2 man pump and release fluid. Still
>>> pedal too soft and not drivable. Removed MC and put on bench and found
>>> front chamber not pushing fluid.
>>> Replaced with another new MC from O'rilly same part number. Pedal
>>> pumped
>>> up tighter but it leaked between the fluid reservoir and cast metal
>>> piston. Still not drivable.
>>> Replaced MC with new one from Autozone (PN NM1641 ). Purchased the Hupy
>>> bleeder. Bleed according to his directions which is RR moving to LF.
>>> Removed lots of air and pumped about 1/2 gallon of fluid through the
>>> system. Still not drivable, pedal too soft and vehicle does not stop.
>>> From 30 to zero takes about 1/4 mile.
>>>
>>> So how do I go about troubleshooting the individual components? How do
>>> I
>>> test hydraulic pressure?
>>>
>>> I'm hoping for someone to give me a systematic approach to resolve this
>>> and get this working.
>>>
>>> Yes, I have spoke with Manny and I really don't think any of the
>>> calipers
>>> are defective.
>>>
>>> Thanks Neil Fonville
>>> 1975 GMC II
>>>
>>>
>>>
>>>
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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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