Brake fluid

[Dek]

Hi Guys
I am experiencing a long brake pedal when the engine is hot which could be due to the effect of the underbody heat on the brake fluid. Is it possible to get a fluid with a higher boiling point? If so what is the specification-eg ? Dot 3/4 etc. and where do I get it? Lastly roughly how much would be needed to completely change it (Jag calipers).
Cheers.
Dek.

[wilf]

oops - double post!

[wilf]

The higher the DOT number, the higher the fluid boiling point. From memoery, DOT 5.1 is the latest, but have a google around to check.
Just avoid going to silicon for now. I think DOT 5 is silicon.

To completely refill your system, a couple of litres should do it, but you might use more purging through.

Also, if you search for "brake fluid" on here, you will find links to sites with the temp info you want.

As I said in my reply to your earlier post, I think there may be other things going on here - boiling brake fluid happens at 300C or higher, I doubt you are doing that in your system. What kind of heat is going into your master cylinder/servo? Do they need shielding?

[dingocooke]

its also worth remembering that the higher the number, the less lubricating properties the fluid has, so stick to DOT 3 or 4 for clutch operating systems if you have an internal hydraulic slave bearing or a heavy clutch.

[Dek]

Thanks for the additional advice on this related second post to my first (not a double post really). I have taken 'on board' the advice given in the first post and think changing the brake fluid must be the first step hence the question in my second post relating to specification and dot no's. What I have found is that Dot 3 has a boiling point of 230 degrees C Dot 4 260 degrees C and 5 and 5.1 even higher. What this means in practical terms god only knows! Also as Wilf says do not mix silicone with glycol based fluids unless a thorough cleaning schedule with methylated spirit is carried out. It seems silicone is ideal for low mileage, infrequently used cars (as a lot of Cobras are) if it is introduced from scratch when the braking system is new. Silicone fluids absorb less moisture from the atmosphere and it is surprising how quickly the glycol based fluids do this. The advice is do not keep the cap off the fluid reservoir for longer than necessary and do not use fluid from a can that has been pre-opened.
Having said all this sometimes it is still comforting to seek advice from fellow owners to see how all this translates into practice and the actual cars we drive (with all their differences)!
Thank you all again. I will let you know the outcome.
Dek

[dingocooke]

In practical terms the temp limit is of very little importance on a fast road car, get it all flushed out and get some fresh DOT4 in it. You might find the extra lubrication of DOT4 will help with pedal weights too as the friction will be a little less, and older brake components with poorer quality machined surfaces are less likely to suffer sticking and other problems running DOT4 as opposed to DOT5. DOT4 is also kinder to the seals, especially 'o' rings.
I wouldnt touch silicone fluid personally.

[Dek]

Originally Posted by dingocooke

In practical terms the temp limit is of very little importance on a fast road car, get it all flushed out and get some fresh DOT4 in it. You might find the extra lubrication of DOT4 will help with pedal weights too as the friction will be a little less, and older brake components with poorer quality machined surfaces are less likely to suffer sticking and other problems running DOT4 as opposed to DOT5. DOT4 is also kinder to the seals, especially 'o' rings.
I wouldnt touch silicone fluid personally.

Thanks Steve
Sound advice. I had intended to replace the fluid with non-silicone but there is also a Castrol glycol based racing fluid with a higher boiling point but then I thought if there is moisture in my present fluid lowering the boiling point the sensible thing to do is replace with ordinary Dot 4 and see what happens!
Thanks again.
Dek

[Sidecarbod]

Originally Posted by Dek

Thanks Steve
Sound advice. I had intended to replace the fluid with non-silicone but there is also a Castrol glycol based racing fluid with a higher boiling point but then I thought if there is moisture in my present fluid lowering the boiling point the sensible thing to do is replace with ordinary Dot 4 and see what happens!
Thanks again.
Dek

If you are talking about Castrol SRF, IMHO its the best stuff you can get, it was the only fluid that I could not boil in my F1 sidecar outfit, and that includes all the stuff with "racing" in its name. (Its ages since I've bought any, I think its only rated at Dot 4 but its wet boiling point is very high and that's what matters)

The major down side is the price, I think it was about 35 quid a litre!

Bit over the top for a road car though

Pete

[Purple AK]

Originally Posted by Sidecarbod

If you are talking about Castrol SRF, IMHO its the best stuff you can get, it was the only fluid that I could not boil in my F1 sidecar outfit, and that includes all the stuff with "racing" in its name. (Its ages since I've bought any, I think its only rated at Dot 4 but its wet boiling point is very high and that's what matters)

The major down side is the price, I think it was about 35 quid a litre!

Bit over the top for a road car though

Pete

Most of us spend £35 on half a tank of fuel! Why would spending £35 on something that could save your neck be over the top

[Sidecarbod]

Originally Posted by Purple AK

Most of us spend £35 on half a tank of fuel! Why would spending £35 on something that could save your neck be over the top

What you say makes a very good point!

I just would be very suprised if you could ever push a road car's brakes hard enough to need SRF. (Especially a fairly light car like a cob)

Your licence certainly wouldn't survive for long, that's for sure

If normal DOT 4 is boiling I suggest that there is something wrong with the fluid, i.e its full of water or air, or there is something wrong with the setup of the car, i.e the exhaust is cooking the master cylinder!

Regards,

Pete