As posted I will be changing my brakes for something bigger
however when I do this I will change the fluid as well
what is the combined wisdom of the best fluid to use
I have seen dot 4 /5/5.1 and synthetic
so whats best ?
NOT synthetic. It will eat your seals both in the clutch and brake system, unless it's been installed from new.
DOT5.1 will be more than adequate. It's also compatible with DOT4, so even if you haven't fully drained the system, you'll be OK. You can also top it up again in an emergency with DOT4. (I haven't manged to make mine spongy with the APs-yet!)
Of course, having drained the system, you'll need a pressure bleeder when you refill. It can take several goes to get all the air out of both systems.
I don't want to seem contradictory, Mark, but the Lucas DOT 5.1 I'm using is synthetic, although I believe there are non-synthetic fluids that also meet DOT 5.1. There are even synthetic DOT 4 fluids available. What is to be avoided is DOT 5, if you can find it at all in the UK, as it is silicone-based and will not mix safely with even the tiniest amount of DOT 3/4/5.1 in your system. I have heard people say it will damage the seals, but I have never seen anyone recommend a change in the type of seal fitted when changing from a glycol-based fluid to a silicone-based fluid, although scrupulous cleaning (i.e. dismantling and cleaning everything with alcohol) is essential to remove all traces of the old fluid. Silicone fluid will coagulate in the presence of glycol fluid.
Anyway, back to the plot. DOT 5.1 is the highest spec. commonly available but there are DOT 5.1 fluids that exceed the spec. by more than others. 5.1 is designed to be less viscous than DOT 4 to help brake modulation in ABS systems.
ATE do a blue racing fluid which I believe will withstand temperatures much higher than will be experienced on the road but I'm told it is very important to change racing fluids very frequently, like after every race.
I'd just like to say that I agree with everyone (for a change!!).
DOT 5.1 is the stuff to use. This spec includes a low temperature viscosity test to ensure that your ABS will work properly should you holiday in Lapland in winter.
If you want REALLY high boiling point fluid use Castrol SRF, if you can afford it, but despite the very high boiling point it is only DOT 4 approved as it gets viscous at low temperatures and so fails DOT 5.1. As SRF stands for silicon racing fluid, most users aren't worried about low temp usage. Note that the silicon in this case refers to its silicon ester base, it is not one of the dreaded DOT 5 silicon fluids which are to be avoided at all costs.
DOT 5.1 can be difficult to find, and several firms produce a "DOT 4 and a half" fluid, eg ATE's "DOT 4 Plus" and Castrol "Super DOT 4". The ATE fluid was produced to meet Mercedes' spec which has always been higher than anyone else's, and the Castrol seems to be some form of marketing idea. Unfortunately, most bottles of the stuff don't give the boiling points so it's difficult to know what you are getting.
So stick to DOT 5.1, if you can find it. Halfords should have it.
There was alot of discussion in the world of restoring Morris Minors a year or so ago regarding the use of silicone brake fluid and copper non-corroding brake lines. The point about not mixing the silicone fluid with "normal" fluid was understood, but not considered a problem because the restoration usually involved fitting completely new brake components throughout.
Silicone fluid was being sold as a long life brake fluid that did not absorb water and so never degraded and never needed changing.
However, the man from Castrol came along and explained that the brake fluid needs to absorb the water vapour that migrates through the rubber hoses etc. This is because it will still migrate through whether the fluid absorbs it or not. So, when using silicone fluid the water stays as droplets in the system. The danger is that the droplets will then freeze in cold weather and if they freeze in the wrong place could seize a brake piston
My handbook for the 9-5 Aero states "top up as necessary with DOT 4 brake fluid. Do not use DOT 5" Also in my 9000CS Carlsson used Castrol DOT 5 something (I think it was 5.1), it was in a bronze coloured can as a top up and within two weeks was having the pleasure of changing the clutch master cylinder as it dumped it's fluid everywhere when the seals went.
Just checked and the fluid I used (twice) in my Carlsson was Castrol Super Disc Brake Fluid DOT 5.1 and on both occassions within two weeks the clutch master cylinder had 1: Given Up and 2: was playing up enough to need bleeding and then replacing. Might be coincence that both times I used a DOT 5.1 fluid in my Carly the master cylinder gave up the ghost soon after but it's been enough to make me very wary of using anything other than DOT 4. Anybody recommend a brake fluid that they use and trust?