At 10:42 AM 2/23/2000 EST, Allen Hefner (Ajhsys@aol.com) wrote:
>In a message dated 2/22/00 7:54:19 PM Eastern Standard Time,
>barneymg@ntsource.com writes:
><< .... maybe I don't like the DOT-4 fluid. .... When I bled the brakes
afterwards the fluid was pretty black with disolved and suspended rubber
particles, .... >>
>....
>Barney, I suspect that the two brake fluids may have interacted to cause
the brake light switch to fail. ....
Well, I'm wide open to speculation on this matter. I've seen many of these
switches fail the same way regardless of what type of fluid is used, and
even when used in the same fluid type forever. I cannot personally
attribute any excess failure rate to to usage with any particular type of
fluid.
I have never opened up one of these switches to see what the internal
construction looks like, but I suspect it would be a thin metal diaphram
with the electrical contact on the top side and the fluid on the bottom
side, with a very thin rubber o-ring seal between the diaphram and the
housing of the switch. I imagine this seal could deteriorate over a period
of many years regardless of the fluid type, with the failure mode being
fluid gradually filling the space on the top side of the diaphram, such
that the air space gets progressively smaller and harder to compress.
Right about now I'm so curious about this matter that I'm tempted to cut
open a new switch for inspection. Before I do that, would anyone like to
contribute an old switch to the cause?
It has been my experience that silicone fluid is a very good preservative
for the rubber bits in a braking system, and as, I would think that
silicone fluid would make the pressure switch last longer than other with
fluids. When I was repacking my brakes after 135,000 miles and 12 years
the rubber cups removed from all of the cylinders looked nearly as good as
the day they were installed, having a nice shiney surface finish and still
carrying all of the original moulding marks and numbers, but with just a
little wear around the working seal edges where they ran against the
cylinder wall. I was quite tempted to put them back in just to see how far
they might continue on without problems, but I was intent on repacking the
entire system to give it a fair comparison while using DOT-4 fluid.
>Could the rubber, or black contaminent, in the fluid be coming from the
brake switch, rather than the new seals?
I don't think so. There was a lot of brackish looking junk in the fluid
all through the system after only 6 months in use, and it repeated again
another 6 months later. I am absolutely sure there is not that much rubber
in the brake pressure switch.
>Was this only found in one wheel line, or throughout the brake system,
which would indicate a particular cylinder or possible the MC?
The gunk was mixed throughout the fluid in the hydraulic system, including
in the clutch slave cylinder (common M/C reservoir on the MGA), so I rather
assume that it was not exclusively a rear wheel cylinder or clutch slave
cylinder problem. I suppose it could be just the front wheel cylinders or
just the master cylinder(s), but as all of the rubber bits came from the
same manufacturer during a short time frame I would think it more likely to
be all of the above. The other alternative of course could be the new
hoses deteriating from the inside, and not the cylinder seals at all. Only
time will tell I guess, and in the meantime I always keep a spare brake
hose in stock (same hose front and rear on the MGA 1500).
>Somewhere at home I have an article written by Lockheed after testing DOT
5 brake fluid and concluding that it should not be used in any system with
a Lockheed brake light switch. If I can find it, I'll let you know.
Yup, someone just sent me a copy of that memo. If it is of sufficient
interest I could forward it to the list.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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