On Wed, 19 Mar 1997, John Cowan wrote:
> The disk brakes on my '60 TR-3A do not release after they are applied
> (with the car up on a rack). Mechanic says that the silicone brake fluid
> used by former mechanic has softened O rings in brake mechanism so that
> they no longer retract the pads when the brake pedal is released.
> Any thoughts or similar experiences?
> Thanks.
> John Cowan
Hi John, I have a couple of comments on your question/problem:
Very often, the cause of this problem is a bad brake hose. The bad hose
is often not apparent from a visual inspection however. As a hose ages,
it can break down on the inside and swell shut. In effect, it forms a
"check valve" that will allow fluid to pass when you press the brake pedal
(extremely high pressure involved here), but not allow fluid to flow the
other way when the pedal is released (the disk brakes have no return
spring or anything to force the fluid back). The result is dragging
brakes. If the hoses on your car have not been replaced in the last 20-30
years, I think this is a likely source of your problem. Incidentally, the
swelling inside the hose can occur regardless of the type of brake fluid
you use, which brings me to my second comment.
Your mechanic's assessment of the situation may well be true (swelling of
seals), but let me make a case against it :-) Early silicone fluid
formulations did have problems with swelling rubber seals, but these
problems have been worked out and you shouldn't encounter this with fluid
produced in the last 10 years or so. Below I've attached a short article
I wrote a few years back to the SOL group about DOT 5. Maybe it will help
in your diagnosis of the problem.
Good luck
Mike Burdick
Omaha NE.
'70 Spit, 5 years on silicone fluid with no problems.
______________________________
A while back when this topic came up, I posted excerpts from a Skinned
Knuckles article on silicone brake fluid. They made the best attempt I've
seen so far to actually address this issue with hard data instead of
anecdotes. The data they used came from military tests in jungle
conditions and tests from the original producer of DOT5 fluid (DuPont? I
can't remember...). Even so, they were not able to come up with many
conclusions about DOT5 fluid. In a nutshell, the conclusions were:
1.DOT5 does not absorb water and may be useful where water absorption
is a problem.
2.DOT5 does NOT mix with DOT3 or DOT4. They also maintain that all
reported problems with DOT5 are probably due to some degree of mixing
with other fluid types. They said the proper way to convert to DOT5
is to totally rebuild the hydraulic system.
3.Reports of DOT5 causing premature failure of rubber brake parts were
more common with early DOT5 formulations. This is thought to be due
to improper addition of swelling agents and has been fixed in recent
formulations.
4.DOT5 is compatible with all rubber formulations.
5.DOT5 doesn't eat paint.
They also made some general recommendations based on this data and
personal experiences. These, along with recommendataions of list members
included:
1.If it works for you, use it. You won't hurt anything if you do the
conversion correctly. (See, #2 above)
2.Careful bleeding is required to get all of the air out of the system.
Small bubbles can form in the fluid that will form large bubbles
over time. It may be necessary to do a series of bleeds.
3.DOT5 is probably not the thing to use in your race car although it is
rated to stand up to the heat generated during racing conditions. The
reason for this recommendation is the difficult bleeding mentioned
above.
4.DOT5 is a good choice for the weekend driver/show car. It doesn't
absorb water and it doesn't eat paint. One caveat is that because it
doesn't absorb water, water that gets in the system will tend to
collect at low points. In this scenario, it would actually be
promoting corrosion! Annual flushing might be a good idea.
|