Topping up brake fluid - what to use

Post by **badfelafel** » Mon Oct 30, 2006 11:41 am

I have front disc brakes and a reservoir (which is low) and want to top up.
Local mechanic says Dot 4 is fine.
Bottle of Dot 4 says not suitable for mineral or silicone based systems.
I dont know what's in the system just now...

Can I use Dot 4?

Many thanks!!

Post by **ColinP** » Mon Oct 30, 2006 12:43 pm

I dont know what's in the system just now...

Can I use Dot 4?

Unhappily, there's only one way to be sure what's in the system - that's to buy a big bottle of DT+OT4 and bleed all the brakes...

I don't think there's a quick way to find out what's in the system, and the loger way (analyst + equiment) is going to take a long time & be costly.

The big question really, is why is the fluid level low? From other threads, disc brake systems need an addtional reservoir so that the front calipers movement is managed by the extra fluid available...

(try adjusting the rear brakes - that may raise the level a bit!)

Colin

Post by **rayofleamington** » Mon Oct 30, 2006 12:51 pm

Unless your car had been previously converted to silicon (very unlikely) then just use Dot4
Generally brakes are never run with mineral based fluid except for a few xceptions (a handfull of French cars)

Post by **alex_holden** » Mon Oct 30, 2006 1:05 pm

If the fluid is purple, it may be silicone. The silicone fluid comes with a label you're supposed to attach to the reservoir, but not everybody bothers to do so.

Post by **JimK** » Mon Oct 30, 2006 1:06 pm

You've got me curious now. What are the properties of the different types of brake fluid?

My mountain bike uses mineral oil but my cars use DOT4. Why would that be?

Post by **Kevin** » Mon Oct 30, 2006 1:30 pm

Hydraulic disc brakes on a push bike, blimey I am out of touch........

Post by **alex_holden** » Mon Oct 30, 2006 1:36 pm

Kevin wrote:Hydraulic disc brakes on a push bike, blimey I am out of touch........

Next thing you know they'll be fitting engines to them...

Post by **JimK** » Mon Oct 30, 2006 1:45 pm

Discs on a bike are good for all the reasons they're good on a car and some reasons that are bike-specific: they are above the mud and cack, and braking no longer depends on the wheel rim being a particular shape.

I've had rim brake pads melt...

Post by **badfelafel** » Mon Oct 30, 2006 1:48 pm

Thanks all... I'll have a go with the Dot-4. If you dont ever hear from me again, you'll know it was a mistake!

Post by **rayofleamington** » Mon Oct 30, 2006 6:05 pm

What are the properties of the different types of brake fluid?

My mountain bike uses mineral oil but my cars use DOT4. Why would that be?

With brake fluid, it's a bit like engine oil - the old ones are superseded and generally not used if there is a better grade. Dot5 is a different animal so for an old car Dot4 is as far as you need to go.
Rather like with an original Minor engine - go for good spec oil but nothing silly like a fully synthetic variant.

Cars generally use brake fluid, not mineral based hydraulic fluid for their brakes due to the robustness of seal materials. Until quite recently, mineral seal materials could not cope with -40°C and > +125°C. Some materials could cope with one extreme but not both even now, mineral seal materials will not survive the extreme high and low temperature (not for long) that is expected from most car braking systems.
Brake fluid seals are generally EPDM based which should cope with this temperature range and up to 150°C

With a mountain bike the chances of these extreme temperatures is minimal therefore the same considerations do not apply.

Post by **bmcecosse** » Mon Oct 30, 2006 7:23 pm

Barry - it's not the end of the world if you add DOT4 to silicone fluid - you will just lose the supposed advantage of the silicone - that it doesn't absorb water. Therefore it doesn't need regular bleeding/throwing away - and the cylinders supposedly don't rust!

Post by **badfelafel** » Mon Oct 30, 2006 8:51 pm

Thanks all - the DOT4 seems to have done the trick
Just need to investigate why it needs topping up!

Post by **bmcecosse** » Tue Oct 31, 2006 7:29 am

Discs do that - the pads wear and the relatively large surface area pistons move out taking some fluid with them. But it would be wise to look for any leaks just in case - most likely is the rear wheel cylinders.