7 replies to this topic

[Mrt88]

Mini city e
1988
998cc

Hi all.
I need to top up my brake fluid now I've replace my rear sub and bled the drums.
My very silly question is I don't know what fluid would be in there currently. Should it be dot4 ?

I've never had to top it up so I don't know. Are there visual differences between types ?
Thanks all!
Theo

[danie garry]

fairly sure it'd be dot4, you can replace the fluid with dot5 if you want but you cant mix em

[Mini 360]

Dot 4

[HarrysMini]

DOT4 is used as standard. However, it can be replaced with DOT5 if replacing the whole braking system (you really don't want to mix them).

 

I would replace it if you're not sure what it is or how old it is. It should be replaced every 2 years.

[dklawson]

The following has been stated above in a couple of ways.  This is an intentional repeat.

 

DO NOT mix DOT-5 brake fluid with any other brake fluid (including DOT-5.1).  DOT-5 must only be used in a system that has been flushed and it is generally advisable to only use it when all the old rubber bits that have been replaced with new.

[HarrysMini]

And don't listen to anyone who tells you that DOT5 degrades rubber seals, it is utter rubbish. 

[dklawson]

And don't listen to anyone who tells you that DOT5 degrades rubber seals, it is utter rubbish. 

 

Agreed... as long as the rubber is new.  The only seals DOT-5 is known to attack are made of silicone and silicone seals are not used in common braking systems.

 

My warning above about not using DOT-5 unless all the rubber bits have been replaced ties back to this.  It is not uncommon for people who say DOT-5 damaged their brake seals to also admit that they did not start with new seals but simply flushed the old brake system and dumped in DOT-5.  With that practice the old rubber that has been exposed to DOT-3 or DOT-4 will "sluff off" and form a gelatinous black goo that collects in brake cylinders followed by leakage passed the seals.  The goo can also hold moisture/water which has been known to pool and cause localized corrosion.   If you are ever considering switching to DOT-5, follow the advice on the net which spells out quite clearly how to flush the metal lines and to replace all the rubber bits with new (including the flex lines).

[tiger99]

Yes, good advice, and the ONLY snags with DOT 5 is that it MUST be used in a clean system, with no trace of any other fluid (including red rubber grease, which is basically just thickened normal brake fluid).or you may get deposits forming which will damage the seals, and it can be very difficult to bleed due to its higher viscosity, and its tendency to entrain small air bubbles. Both of these problems can be overcome, and when I have a car old enough to need a hydraulic overhaul, I always replace the lot, using Goodridge hoses, Cunifer tubing and new cylinders, or new seals as needed. I always dismantle and clean new cylinders thoroughly with methylated spirit, to get rid of the grease, and re-grease with silicone high temperature grease, then assemble and fill with DOT 5. No hydraulic maintenance will be needed again for a long time.

 

What you gain is a higher boiling point, and no water absorption, so no risk of brake failure shortly after a few hard brake applications, using old DOT 4, etc, which has absorbed water. Detractors say that there is a serious corrosion problem, because water vapout enters via the master cylinder vent (true, but you could fit a dessicator), and being denser than the fluid, finds its way to the lowest point of the system (also true) which, they claim, is the wheel cylinders or calipers (almost always not true). A few globules of water in the fluid, lying in the horizontal run of Cunifer under the floor, is not going to harm anything.

 

My opionion is that only if you are very confident of being able to bleed the system, and it is all new and has been cleaned, use DOT 5, otherwise stick with DOT 4. I would advise a label on the master cylinder (they are available from kit car suppliers as they are required for IVA, and I think they are mandatory on new cars), just to ensure that no-one tops up with the wrong fluid, or even a well-meaning garage duiring the MOT does an extra test (which I know some do, from personal experience), inserting a boiling point test probe that is contaminated by DOT 4 from a previous vehicle. If DOT 5 is in use, the boiling point test is best just not done, as it is not needed.

 

My experience of DOT 5 in several vehicles, from a Mini to an Avenger and Cavalier, possibly also an Opel Rekord, but my memory of that horrid and unreliable car is a bit vague, all doing phenomenal mileage, is that DOT 5 is very good indeed, if your system meets the requirements for using it.

 

Edit: As I was composing this, Doug explained rather well why you get the nasty deposits on the seals if you don't do something about it.

 

Another edit: I should have said that water can also enter the system through the rubber hoses, which are permeable to water vapour, which is one of the reasons why I always use Goodridge hoses. PTFE is impermeable to almost everything, including water vapour.

Edited by tiger99, 07 May 2014 - 12:13 PM.