At 02:02 PM 2/23/2000 EST, Allen Hefner (Ajhsys@aol.com) wrote:
>....
>The three problems with silicone are that,
>1. it does not absorb water. This allows water to pool in the lines
unless you bleed it out every year. This is probably a smaller problem in
the desert where hunidity is non-existent.
I live in a fairly humid are near Chicago, Illinois, and I drive my MGA in
all kinds of weather, and it gets soaking wet pretty often. Check this for
example:
http://www.ntsource.com/~barneymg/recent/ccfg0599.htm
Until recently I have been using silicone fluid, and I have flushed fresh
fluid through every two years like clockwork. When I opened the system for
a repack after 12 years and 135,000 miles all of the cylinders were in good
enough condition to be lightly honed and repacked, none of them had to be
replaced. To me that can only mean that there is little or no water
settling in any of the cylinders, which is where it usually discloses its
presence by way of pititng in the bottom of the cylinder bores.
>2. it is (or was in original formulation) just a bit more compressible
than DOT 3 or 4 brake fluid. .... Barney, you use that A for some serious
autocrossing. Did you notice any difference in pedal feel between DOT 5
and DOT 4? And 3. the above seal swelling problem.
Absolutely no difference in performance or pedal feel with DOT-5 or DOT-4.
Years ago while using DOT-5 I was pushing it pretty hard downhill an a
twisty mountain road until the drum brakes started to fade a bit. The
pedal feel didn't change at all, just that the braking effect started to
diminish. This is typical of drum brakes, as the thermal expansion changes
the shape of the drum from cylindrical to slightly conical as the flat
mounting side of the drum keeps one edge from expanding as much as the
other side. When this happens the brake shoes do not firmly contact the
full width of the drum, and the smaller part of the shoe still in intimate
contact with the drum sees a higher pressure (in psi), and gets still
hotter yet, which is what causes the fading. If you can just stay off of
the brakes for about one minute the fade should go away and full braking
effect be restored. As the pedal feel did not change throughout this
incident I figure the silicone fluid was not affected at all by the
increased temperature of the braking system.
As a side note, disk calipers are somewhat more suseptible to absorbing
heat from the brake pads than are drum brakes, so even though the disk
brakes are less prone to fade they may be more likely to suffer from the
effects of hot fluid.
Now less than a year ago, after I had switched the system to DOT-4 fluid, I
had my MGA on the track at Blackhawk Farms Raceway (2 mile circuit) running
flat out hot laps for 45 minutes straight on race tires (top speed about 70
mph at the end of the longer straights). During this time I had absolutely
no noticeable fading of the brakes or any change of pedal feel, but this is
obviously attributable to the fact that there was only about 10 seconds of
heavy braking on each 2 minute lap, only 1 or 2 seconds at a time here and
there in several places around the track.
So IMHO, either DOT-5 or DOT-4 fluid is easily up to the task of operating
the brakes on an MG in any normal driving condition, and probably most
racing conditions as well. My primary reason for using DOT-5 was because
it doesn't eat paint if there's a leak or a spill. As I have never
encountered either case in many years and many miles with my (current) MGA,
I have in recent years come to discount that quality considerably, and now
have enough confidence to use DOT-4 without reservation. That doesn't mean
I like it (yet), just giving it a fair try for comparison sake. Will let
you all know in another 10 years or so.
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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