Silicon Brake Fluid

[Chassis 33]

I've got a car that came with DOT5 (silicon) fluid - I know that if you go from standard to DOT5 you have to change all the seals hoses etc to stop them swelling up, disintegrating and generally causing the end of the world, but somewhere in the back of my mind says you don’t have if going from DOT5 back to a standard fluid, just flush the system through and Bob's your auntie's husband.

Is this right or have the ravishes of time addled my memory?
Regards
Iain

[//j17]

EARLY silicon fluids were not compatible with mineral fluids and required a full seal change/system flush, but that's not been the case for a while now.

The most common DOT5 fluid in the UK is Automec and they clearly state on the bottle that it's 100% compatible with mineral fluids and no seal changes are required*. They advise against mixing silicon/mineral fluids but just because the blend ends up with the worse properties of the two - the lower boiling point and slightly spongy pedal of silicon and water absorbency and paint-stripping of mineral.

• That said if you are going to drain all the fluid it's rather silly to not spend a few extra pounds on changing all the seals. They don't last for ever and you don't want to have drain the (comparatively expensive) silicon fluid to change the master cylinder seals when the next MOT test comes around and the tester tells you the m/c is leaking.

[Mr2Mike]

//j17 said:

The most common DOT5 fluid in the UK is Automec and they clearly state on the bottle that it's 100% compatible with mineral fluids and no seal changes are required*.

That's a very impressive claim for a silicone fluid, is is also compatible with the far more common glycol base fluid? Given such a unique claim I wonder why they don't mention this on their web site?

[Scuffers]

sounds more like you are talking about DOT 5.1 -NOT silicon (DOT 5)

you cannot mix DOT5 with anything else, and it's not to be used in systems not 100% designed for it.

DOT 3,4 & 5.1 are all 'mixable'

5.1 is often called synthetic, however, it's NOT silicon.