<VV> Compressibility of DOT-5 (Silione based brake fluid)

Hugo Miller hugo at aruncoaches.co.uk
Sun Jan 27 16:32:07 EST 2019

Or just stick with the old stuff & save all the bother? ;-)
On the subject of DoT numbers, there is plenty of scope for confusion. 
My understanding, and I could be wrong, is that DoT 4 is the same as DoT 
3 but with a higher boiling point? DoT 5.1 is the same stuff but higher 
temp still? And DoT 5 is the completely different silicone stuff. Is 
that correct?
I also have a bottle of 'synthetic' brake fluid somewhere, whatever 
that may be.

On 2019-01-27 15:42, Jim Simpson via VirtualVairs wrote:
> There is some basis for the idea that DOT-5 based brake fluid is 
> somewhat
> compressible.  From what I've been able to determine, the issue is
> entrained air, not the fluid itself.  If air is somehow mixed in 
> (from
> shaking the can or not pouring it very carefully), it takes a very 
> long
> time for the tiny air bubbles to come out.  Much, much slower than 
> from
> DOT-3 fluids.
> It's the bubbles that compress!  There might some slightly greater 
> fluid
> compressiblity of the DOT-5 than DOT-3, but the entrained air is 
> frequently
> the issue.
> The moral of the story is to be very careful when filling a system 
> with
> DOT-5 brake fluid to ensure the fluid isn't shaken or capturing air 
> as it
> is poured.
> Jim Simpson
> Group Corvair
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