Brake Fluid for E46 M3?

[robroy123]

Manual says DOT 4 and halfords say DOT 5.1. Mine needs a top up after 3 years use.

[m12_nathan]

It is replaced evey 2 years anyway? Little clock symbol above the service countdown when it is due.

I use AP PRF 660 which is a super DOT4 fluid and seems pretty good, probably not quite up to Castrol SRF standards but from memory about 1/2 the price.

[Mroad]

Brake fluid needs changing every two years max, if yours hasn't been changed in that time get it done. Brake fluid is hydroscopic and absorbs moisture leading to a spongy pedal under braking (the fluid boils at a lower temperature with moisture absorbed in it) and corrosion of the calipers.
The fluid will only need topping up if it has dropped below the minimum mark, if it has check your brake pad wear as that is the only reason your fluid level will drop unless there is a leak somewhere (which obviously needs tracing). If you do top up the fluid and then later change the brake pads, pushing back the pistons into the calipers will just make the fluid overflow the reservoir so its pointless to top up unless you really have to.
DOT levels can be a bit misleading as some DOT4 fluids can have better performance than DOT5.1, they just don't meet the DOT5.1 viscosity values (introduced for super critical ABS systems). If the manual says DOT4 then use DOT4. I use Motul RBF600 (DOT4), ATE Racing/Super Blue (DOT4) is also good for long term use (and I believe was the stock BMW fluid for a while).
Whatever you do, do not use DOT5 fluids. They are silicon based and are incompatiable with DOT4 and DOT5.1 fluids (and they have rubbish performance as well).

[robroy123]

Thanks all for your advice ... I will get the brakes fluid changed at the main dealer. Anyone else have brakes that grind under heavy breaking as mine have done it from new?

[robroy123]

Thanks all

Does any have any experience of using PCS (performance Car Specialists) in Milton Keynes, as I want to avoid my local BMW dealership!

Would like to get my brakes done properly, new pads etc . Oh and are M3 brakes supposed to sound like wild thunder under heavy braking?

[buffly]

M3 brakes do make a `howling` noise when they get warm, this is produced by the pockets of air getting trapped in the holes of the disk when the pad travels over them. Produces a `popping` sound, and at high speed, sounds like a `howl`! There is no fix for this i`m afraid. We get a few people asking about this at my dealership, but nothing can be done....

[sjwb]

May I clarify a couple of points?
The adjective is hygroscopic (capable of easily absorbing moisture)which in itself does NOT cause a spongy pedal. The sponginess is more typically caused as the moisture boils off when the fluid exceeds 100 degrees Celsius - thus leaving a vapour pocket. As we all know gases can be compressed where fluids may not.
Believe it or not, over time, an amount of moisture is absorbed through brake flexi hoses!