Home » Public Forums » GMCnet » [GMCnet] Disc brake project still in failure mode
| Re: [GMCnet] Disc brake project still in failure mode [message #282405 is a reply to message #282402] |
Sat, 18 July 2015 14:17   |
Neil Fonville
Messages: 68
Registered: May 2014
Karma:
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Member |
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Yes, I have the P-30 MC and the front port feeds the front brakes, etc. I
do have a flexible type hose between the bogies. They are steel braided
but certain they are rubber on the inside.
In response to Ken’s comment: I took the readings after holding the pedal
down for a few seconds or so and continued to hold for about 1 min. They
are were very consistent and did not see any movement of the needle during
the reading.
If I still had some air somewhere in the rear, would I see a lower reading
at the caliper than the MC? I would think do. I wanted to take the
reading at the MC to rule it out. This is my 3rd one from O’rilley.
On 7/18/15, 1:41 PM, "Gmclist on behalf of James Hupy"
wrote:
> Neil, one more thought crossed my mind. Do you have a flexible line
> between
> the forward and rear bogies, or are they rigid ones. I have seen both
> types
> used. The possibility exists that you have an internally collapsed flex
> line, or, a pinched or kinked rigid line there. It seems highly improbable
> that hydraulic pressure would vary much from one end of a line to the
> other
> end, or, in the case of the rear brakes, from side to side. All the rear
> brakes are fed from a single line. If you do have a P-30 master cylinder,
> you had to reverse the two lines that go to the distribution valve to make
> the fittings match the threaded ports in the MC. That would make the
> reservoir towards the front of the coach feed the front brakes, and the
> other one feed the rears. The pv4 should have enter and exit pressures
> that
> are the same. There is nothing inside the valve to alter pressure or flow.
> The pressure differential, if any, should activate the spool valve and
> close the brake warning light circuit. There should be equal pressure on
> both sides of that spool, or it will move one way or the other and operate
> the warning light.
> Jim Hupy
> On Jul 18, 2015 11:10 AM, "A." wrote:
>
>> Neil Fonville wrote on Sat, 18 July 2015 12:09
>>> I did not read the front pressure because the fronts are solid and
>> seem
>> to be working. Since I had the front working, no leaks, etc I did not
>>> want to open that part of the system up. Not really sure why that
>> would
>> tell me more? 800-900PSI at the MC and 300-400PSI at the caliper. Huge
>>> loss somewhere in the rear system and the combination value seems
>> likely. From what I have read, the PV2 value will cut the pressure by
>> 1/2
>> within
>>> a certain range. I¹m thinking I got sold the PV2 instead of the PV4
>> when I replaced it.
>> It is understandable that you don't want to open up the front since it
>> is
>> working. But I have made the mistake of thinking the line farthest from
>> the
>> booster is the rear and the other is the front, which was backwards (or
>> vice versa, I have to trace the lines to tell, since I can never
>> remember).
>>
>> That's why I asked if you are sure the line you checked at the master
>> cylinder is actually the line to the rear brakes. I don't know what can
>> happen
>> to cause 800 psi at one end, and half that at the other end. Doesn't
>> seem
>> plausible if they were the same line.
>>
>> If you have the OEM type combination valve, at light pedal pressure you
>> can have less pressure to the FRONT than the rear. And in that case, you
>> could
>> have less pressure to one set than is coming out of the master cylinder.
>>
>> To summarize the thought process here: Something LIKE what you are
>> describing COULD occur with an OEM type combination valve, and the lines
>> from that
>> valve that are supposed to go to the front going to the rear, and vice
>> versa. But not the numbers you are reporting
>>
>>
>>
>> http://www.gmcmhphotos.com/photos/deciphering-the-combination-valve/p5331
>> .html
>>
>> There are also cases where folks had to reverse the lines at the master
>> cylinder to get the fittings to work (because one is larger than the
>> other).
>> --
>> '73 23' Sequoia For Sale
>> '73 23' CanyonLands For Sale
>> Upper Alabama
>> On "marriage equality": Once you define the direction that society is
>> headed as "forward", you can call it progress.
>>
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1975 GMC II
Allen, TX
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