Brake Fluid Query

[mazdajason]

Evening all.

OH has just come home and said her handbrake light had come on while she was driving home. No loose connection or anything so i looked at the brake fluid level in the engine bay and yup its low enough to get the sensor to come on.
So away to halfords for dot3 brake fluid like it says on the fluid resoivoir (speeeling?) lid but they dont have any, only 4 and 5/5.1 . Tesco dont have any either!
Am i alright putting in dot4 or is the boiling point too low/High?
Cheers all.
Jason

[skinny]

either DOT4 or DOT5.1 are fine and perfectly compatible.

[mazdajason]

That's great thanks a lot!

[bluetone]

Check calipers/hoses for leaks?

Edited by bluetone on Thursday 30th September 05:53

[mazdajason]

I will do that as well, are there sensors for when the brake pads are low? thats another thing that can fire up a warning sign on the dash too. Just not sure if the MX5 has the sensors or not?

[MX-5 Lazza]

No pad sensors. There is a screech clip that will touch the disc when the pads are very low and make a lot of noise but don't rely on that - they should be changed well before that happens.

[bluetone]

^^^^ AFAIK the only things that will activate the brake light are the handbrake and low fluid in the reservoir.

[tristancliffe]

Don't use DoT5 fluid - it's Silicone based (and rubbish), and is NOT compatible with DoT3/4/5.1

[mazdajason]

Cheers all, topped up with dot4 yesterday and teh light went off instantly.

[youngsyr]

Whilst we're on the topic of brake fluid, do people find much value in using high spec. fluid, such as Motul RBF 600, for fast road/track, or is bog standard DOT 5.1 good enough?

[MX-5 Lazza]

I keep meaning to get some high quality stuff but to be honest I haven't had a problem with the cheap Halfords 5.1 I've been using the last 4 years or so so it's one of those things I'll probably never get around to doing...

[youngsyr]

MX-5 Lazza said:

I keep meaning to get some high quality stuff but to be honest I haven't had a problem with the cheap Halfords 5.1 I've been using the last 4 years or so so it's one of those things I'll probably never get around to doing...

Do you ever find that the brake pedal travel gets a bit longer before the brakes bite after a few laps?

[MX-5 Lazza]

youngsyr said:

Do you ever find that the brake pedal travel gets a bit longer before the brakes bite after a few laps?

Nope. The only reason it will do that is if the fluid is a bit old. Brake fluid is hydroscopic i.e. it will absorb moisture. If there is too much the moisture will boil causing bubbles in the fluid and this in turn will cause a soft pedal.

[youngsyr]

MX-5 Lazza said:

youngsyr said:

Do you ever find that the brake pedal travel gets a bit longer before the brakes bite after a few laps?

Nope. The only reason it will do that is if the fluid is a bit old. Brake fluid is hydroscopic i.e. it will absorb moisture. If there is too much the moisture will boil causing bubbles in the fluid and this in turn will cause a soft pedal.

On Evos you can boil even fresh standard fluid on track if you have uprated pads and discs, you can feel it's about to happen because the pedal travel gets longer and anyone with any sense will use a high quality track orientated brake fluid.

I was wondering whether MX-5s ever have the same problem, but appears not, so I'll save the cash.

[skinny]

i used to use Motul RBF660 as it's a pretty cheap fluid (aboutt twice the price of a fairly cheap standard DOT4).

now i use brembo racing brake fluid as i got some free. unless you're a total track monster tho, Dot5.1 should be fine

[norwichphoto]

Track or racing brake fluid has a very high dry boiling point, but when it gets contaminated it has a very low boiling point, which is why it often just has a DoT 3 racing.

Fluids get changed very regularly on a race car - eg after every race weekend.

DoT 5.1 for road use or Castrol Super Response DoT 4

[skinny]

actually if you check the spec of something like motul RBF600 the dry boiling point is 312 deg C, wet boiling point is 216 deg C.

compare that to the minimum specs of a DOT5.1, which is higher than DOT 3 or 4, at 270 deg C and 190 deg C dry and wet.

[youngsyr]

skinny said:

actually if you check the spec of something like motul RBF600 the dry boiling point is 312 deg C, wet boiling point is 216 deg C.

compare that to the minimum specs of a DOT5.1, which is higher than DOT 3 or 4, at 270 deg C and 190 deg C dry and wet.

Agreed, I discussed this years ago on another forum and from memory it was something completely unrelated to the practicalities of using track orientated fluid in road cars that made it have the lower DOT rating?


ETA: Found the discussion:

me ages ago said:

I believe the Motul RBF 600 meets all the DOT 5.1 requirements except cold viscosity, and therefore is rated as DOT 4, but it far exceeds the DOT 5.1 ratings for both wet (i.e. when the fluid has been in a while) and dry (when the fluid is brand new) boiling temperature, which is what counts on the track.

Edited by youngsyr on Thursday 7th October 09:38

[skinny]

yep, the boiling specs are a minimum - so whereas DOT5.1 has higher minimum wet and dry boiling points, it also differs from DOT 4 in it's viscosity, it's thinner when cold which is needed for modern ABS systems. this is why even when you have a racing brake fluid with very high boiling points, it's almost always still a DOT 4, because of the viscosity.