Yeah, what is wrong with silocone?
There is really nothing "wrong
" with Silicone (Dot-5 NOT
) - if ... the system is specifically
designed for it, as are many higher end performance vehicles.
The problems arise when folks start mixing early hydraulic design components with the different fluid characteristics of a Dot-5 system.
Most common 'short-term' issues are due to components sizing leading to "spongy" pedal feel and braking bias calibration(s) which takes time, and familiarity with, to overcome until a good balanced feel can be achieved.
You can select components during system construction to minimize this if your building/rebuilding a vehicle braking system from ground up.
OEM systems using this fluid are specifically engineered from the 'get-go' to work with these characteristics.
Early glycol based (Dot 5.1) fluids were also not fully compatible with many of the elastomers (you know - rubbery stuff
) commonly used in seals and internal o-rings.
The glycol-ethers can/would attack the components over time and leaks were the common indicators of 'issues' ...
The true 'silicone' fluids don't exhibit this, and most components are now multi-fluid compatible from the vendor, so this is not as big an issue as earlier - but still a concern depending on your systems history and composition.
Lastly - silicone fluids are completely 'non-hygroscopic' (can't absorb any water
That sounds like a "good-thing
Wellllll .... not so much
It depends on where in the system any entrained moisture (oh it will get in there, trust me
) comes to rest and it's concentration(s).
It will most likely settle to lowest points in the system - think calipers here - and when it does it then is in the worse place it can be, right where the maximum heat zones are in the system. And since water has a very low boiling point you can probably guess what happens to your braking effort as you get into spirited braking cycles and the caliper temps exceed 400+ degrees
[Note: the earlier Glycol based (Dot-5.1) fluids just absorbed some of this moisture - so even though they were hard on seals, they didn't have the lost of effort issue 'true' silicones do.]
On true performance and/or Race systems, the fluid is bleed, checked, serviced and/or changed very often during routine vehicle prep cycles - so these guys don't see much if any problem with moisture entrainment.
Hope this helps understand the 'why' most suppliers don't recommend the "DOT-5" type fluids.
And ... I'm sure you'll most likely never exceed the limits of a good DOT-4 anyway, so there should be no incentive to 'pioneer' use of silicone on the majority of street driven vehicles.
Oh, wait a minute ... I just thought of a GREAT use for Silicone