RE: [MV] Silicone Brake Fluid & Rubber

Alan Bowes (
Mon, 1 Dec 1997 16:24:35 -0700

On Monday, December 01, 1997 12:52 PM, Robert Bell
[] wrote:
> My response is not about the use of brake fluid to remove paint, but
> about the problem of some brake fluids dissolving rubber;
> particularly silicone brake fluid. In the past there has been a
> rather long thread on the merits of using silicone brake
> fluid in MVs. After reading those messages I had decided to
> use silicone fluid in my '43 Federal truck in which I am
> replacing all lines, cylinders, etc.
> This past week, however, I contacted a couple of brake master
> cylinder rebuilders about rebuilding a master cylinder for
> my truck. Dapp Enterprizes' year-long guarantee is
> conditional upon NOT using silicone brake fluid, stating
> that it turns some rubber to gel. They've had some
> re-built cylinders returned within three days all gummy
> & gooey due to silicone fluid being used.
> White Post Restorations also does NOT recommend the
> use of silicone brake fluid. Billy Thompson (owner) states
> that it may work fine in 9 out of 10 cases, but really creates
> a mess sometimes. They used silicone in a few vehicles
> which they restored, but soon abandoned its use because
> where it dissolved the rubber, the entire brake fluid
> system had to be replaced. Plus, he states that it is more
> difficult to bleed the wheel cylinders and gives a spongy pedal.
> I have abandoned my intention of using silicone brake fluid in my
> truck's braking system.


>From what I've read, the reports of DOT5 dissolving rubber brake parts
were far more common with early DOT5 formulations, due to the inclusion
of swelling agents to keep seals pliable. I've heard that this has been
fixed in recent formulations. As to spongy feel, DOT 5 is slightly
compressible, but any sponginess should really only be noticeable at
high temperatures, when it becomes much more compressible. Most reports
of sponginess probably come from cases where air bubbles have been
trapped in the fluid. As you mentioned, it is more difficult to bleed,
because it can trap air bubbles more easily than the glycol fluids.
Bleeding should be done slowly, and DOT 5 should also be poured very
gently into the master cylinder reservoir to avoid trapping bubbles.


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