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| Re: [GMCnet] Disc brake project still in failure mode [message #282402 is a reply to message #282401] |
Sat, 18 July 2015 13:41   |
James Hupy
Messages: 6806
Registered: May 2010
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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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