Copper brake pipes and Dot4

[Trumper]

Hi

Just bled the front callipers after overhauling all the front suspension on the GT6 and filled topped up with Dot 4. The brake pipes are copper but i never gave the contents of the reservoir a second thought

Now im wondering if the previous owner filled up with silicone fluid.

If so i have a couple of questions

1) Whats the issue with mixing the two?
2) What effect if any does Dot 4 have on the copper pipes if any?

I think i might just drain the system for safety’s sake and refill with new Fluid - any advice on which fluid?

Thanks - Andy

[CharlieB]

The two fluids are a different colour. Silicone is purple when new & clear when used, DOT 4 is slightly yellow. I would just bleed through the whole system with DOT 4 untill you get nice clean fluid coming out. The two types don't mix.
DOT 4 is fine with copper pipes.

[Llessur]

Am certainly no expert but as far as I know you definitely can't mix silicone (DOT5) and mineral (DOT3, 4, 5.1) fluids.

I also have heard it's not a good idea to use silicone fluid in an old system that has previously used mineral fluid as it can cause blockages/sticking pistons etc.

Would stick with mineral unless you think it's been changed - if it ain't broke don't fix it. If you're not sure if you might have mixed the two then probably a good idea to rebuild the master cylinder and pistons with new seals etc - or at the very least give it a good flushing with one or the other type of fluid. I guess if you have mixed the two and the mixture's not been in there very long then a good flushing with one or t'other could do the trick if you're after a quick fix.

Think silicone is pretty difficult to get rid of once it's in the system so you if you're not up for replacing the seals you could give it a good flush with silicone and use that from now on and hope for the best. Might cause more problems if you changed back to mineral once silicone's been in the system. I'd change the seals and go back to DOT 4/5.1 though - but then I'm a scaredy cat when it comes to brakes.

Not aware of any particular problems with DOT4 fluid and copper pipes. But then some say I'm fairly ignorant...

[Mj17]

Silicon and mineral fluids CAN be mixed (part of the DOT specification) but it's a waste of money to do it as you get the worst of both fluids - slightly spongy pedal/lower boiling point of 5 + water absorbtion of 4.

The early DOT5 silicon fluids did come with a recommendation to change the seals but the modern Automec stuff states clearly that this isn't necessary (though if your draining your fluid it's a logical preventitive step!).

[CharlieB]

Silicone fluid is much less dense than mineral fluid so rises to the top (& any water sinks to the bottom) so they don't actually mix. That's not to say a brake system with both in wouldn't work but I agree its not a good idea.