In a message dated 2/22/00 7:54:19 PM Eastern Standard Time,
barneymg@ntsource.com writes:
<< When the system was repacked I switched from silicone fluid to Casterol LMA
DOT-4 fluid, just out of curiosity. Shortly thereafter the brake pressure
switch failed. I wouldn't say that had anything to do with the DOT-4
fluid, any more than it had anything to do with the DOT-5 fluid. It was
just time for the switch to expire, that's all.
Otherwise, I think maybe I don't like the DOT-4 fluid. After about six
months I had to R&R a short section of steel brake line at one front wheel
while I was replacing a worn lower swivel link. When I bled the brakes
afterwards the fluid was pretty black with disolved and suspended rubber
particles, so I flushed the whole system. Because of this, about six
months later I bled the brakes again just to check, and sure enough the
fluid was quite dirty again, and so I flushed the entire system again. I
never had this problem with the silicone fluid, so naturally I'm a litle
concerned about this. Would anyone care to venture a guess whether this
condition may be an effect of the DOT-4 fluid, or maybe some inferior
rubber products having gotten into the system with the last rebuild? And
FWIW, all the rubber hydraulic parts came from Moss Motors. >>
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First of all, I want to say that there are very few opinions on this list
that I respect as much as Barney's. He will forget more than I will ever
know about British cars.
Barney, I suspect that the two brake fluids may have interacted to cause the
brake light switch to fail. One of the problems is the differing amount of
fluid that can be absorbed by the rubber bits when switching fluids. I
believe the problem as stated when DOT 5 first appeared, was that DOT 4 fluid
(originally supplied with the car) caused the seals in the brake light switch
to swell, and switching to DOT 5 allowed them to shrink and leak. I haven't
checked the archives, but that is my recollection. Since the fluids cause
swelling at different rates, that may be why the switch failed, even though
you went from DOT 5 to DOT 4.
Could the rubber, or black contaminent, in the fluid be coming from the brake
switch, rather than the new seals? Was this only found in one wheel line, or
throughout the brake system, which would indicate a particular cylinder or
possible the MC?
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.
Allen Hefner
SCCA Philly Region Rally Steward
'77 Midget
'92 Mitsubishi Expo LRV Sport
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