At 02:23 PM 3/9/2002 -0800, you wrote:
>Duncan,
>
>Yes, now that you mention it, that does ring a bell. I believe the early DOT
>5 fluids would not swell the rubber at all and had leakage problems. I
>remember someone (Tom?) suggesting soaking the seals in DOT 4 for a few
>hours before assembly to "condition" them. I've heard that the newer DOT 5
>fluids have an additive that swells and softens the rubber. I wouldn't
>consider actual fluid leakage as a cause for "sponginess". I think you can
>feel the difference and if it's bad enough to be noticeable to the foot,
>you'll know it's a leak pretty darned quickly. If you had soft pedal when
>the brakes were cold, then you had air in the system. If they got soft after
>they were warmed up, then you had moisture.
>
>Bob
Yes, that was me, the other Tom with Tigers. I don't really
remember precisely which brake fluid I used to swell the seals, but it's a
good bet that it was Castrol. I discovered this effect quite by accident
when rebuilding some brake components, The British rubber rebuild parts
were set to soak in a bowl of type 4 fluid and something came up and I was
diverted for a while. When I returned several hours later, I found the
rubber components severely swelled (+25%).I was familiar with the reaction
to some rubbers to the swelling effect of aromatic (help me here, Organic
Chemistry was a long time ago) hydrocarbons and this was exactly the
reaction I observed. I know from experience that this is a reversible
process, but it was important to understand that the design basis of these
seals was that they would swell a certain percentage after they were
installed. This was critical to the use of Silicone brake fluids of that
era, as they did not have any additives to produce this effect.
Thereafter, I used a 20 minute soak in Castrol/Girling, followed by a wipe
clean to avoid contamination, prior to assembly of my brake components. I
have used Silicone for about twenty years and have never had any of the
problems that are often reported. I don't think that there is enough
exposure within the system to allow the seals to loose the small swelling
that they develop during this short exposure to the Girling
fluid. Admittedly, my cars haven't seen track time on an open track type
of event, but on the street, they give no problems. In fact, Peter Thomson
and I rebuilt the front calipers on the White Tiger just before TU last
year, and I built that dual cylinder system in the late 70's. I used the
same procedure and DOT5 fluids on both the Blue Tiger and Rob's Tiger, and
both have autocrossed without brake problems to date. I do however agree
with your analysis of sponginess, as it relates to when the problem occurs.
The Tiger system in general and the various methods used to "bleed" the
brakes still leave room for improvement. Pressure bleeding is the best
method I know of. You can keep all your vacuum tools (toys), cause they
all tend to leak unwanted air into the system. It's just that the pressure
system tends to spray fluid all over, and unless your using DOT 5, Your
paint is going to tell you where it went.
Tom
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