Brake fluid

[SimonBiggs]

Apart from being fed up with the number of so called expert parts and products suppliers who think Jaguar only made one S Type in 1999, my question is simple. Which brake fluid, Dot 3 or Dot 4 do I use in my 1967 S Type 3.4 auto.
Also, I assume that I need to completely drain the system before adding a synthetic fluid to the existing fluid which I assume is alcohol based?
Many thanks in anticipation.
Simon.

[Orlando St.R]

Hello, Simon, and welcome to the forum.

Yes, I have similar frustrations about the later S-type. It also makes a simple parts search on ebay such a pain. There should be a law against manufacturers re-using names from their glory days.

Still, I always use DOT4 in my S-type. I think you would be wise to completely drain the system before switching to synthetic.

Glyn will be able to give you chapter and verse.

Out of interest, I came across these cheap brake fluid testers, which tell how much water has been absorbed by the fluid (which will presumably be none in the case of synthetic fluids). It saves repeatedly changing the fluid needlessly in a car that isn't use very often.

https://www.ebay.co.uk/itm/Car-Brake-Fl ... 3708044503

brake fluid tester.jpg (145.36 KiB) Viewed 2752 times

[NigelW]

Yes Simon you can use Dot 4, the benefit over Dot 3 is the higher boiling point. If you want to go down the Dot 5 silicone route then it is best to install on a completely fresh brake system as any previous trapped fluid would would still give problems. I've been using silicone in my S Type since the 1970's with no problems.

[cass3958]

Thats a nice cheap tool for my tool kit Orlando. Will have to order one.

[Glyn Ruck]

Dot 4 Brake fluid is ideal for our S Types. They are polyglycol & polyglycol ether based. Brake fluid is hygroscopic & thus should be flushed approx every 2 years. Exotic brake fluids are not necessary & can give one elastomer compatibility issues. That is a neat little device Orlando shows if it is relatively accurate. As water level rises in the fluid so boiling point reduces leading to brake fade. At least we don't have ABS systems to worry about where corrosion byproducts can severely damage the system. Nevertheless as we have single circuit brakes with no backup other than the handbrake we should keep these systems in fine fettle & free of corrosion.

Buy brake fluid in small containers. Only unseal immediately before use. Never reuse brake fluid. Don't use brake fluid that has been stored for long periods after unsealing.


I have had all clutch & brake cylinders & calipers stainless sleeved with stainless pistons in the calipers on my car.

[Orlando St.R]

Thanks for the info and advice, Glyn.

[primaz]

Dot 4 Brake fluid is ideal for our S Types. They are polyglycol & polyglycol ether based. Brake fluid is hygroscopic & thus should be flushed approx every 2 years. Exotic brake fluids are not necessary & can give one elastomer compatibility issues. That is a neat little device Orlando shows if it is relatively accurate. As water level rises in the fluid so boiling point reduces leading to brake fade. At least we don't have ABS systems to worry about where corrosion byproducts can severely damage the system. Nevertheless as we have single circuit brakes with no backup other than the handbrake we should keep these systems in fine fettle & free of corrosion.

Buy brake fluid in small containers. Only unseal immediately before use. Never reuse brake fluid. Don't use brake fluid that has been stored for long periods after unsealing.


I have had all clutch & brake cylinders & calipers stainless sleeved with stainless pistons in the calipers on my car.

Glyn,

I know you are very knowledgeable on lubricants, etc. So even with extreme hard driving as I sometimes do, what would you recommend for brake fluid? The reason why I ask is that I recently went on a drive with a lot of more modern performance street cars and the drive was 118 miles of almost all 15-45 mile an hour curves where we were driving very fast on the limited straights and double the posted speed on the curves. My car with the V8 was able to accelerate hard but then I would use the brakes hard especially since my car is an auto and about 70% thru the almost 3 hour drive where we were averaging 50-90 mph my brakes faded and I had to back off and just let them go ahead and take my time; towards the end my pedal went down a lot more likely due to the fluid boiling over as after lunch the brakes were back to normal pressure. Many of the others even had their modern brakes smoke due to the abuse. I think I will take one of my sports car on their next run if the route has that many curves but the old Jag did hold its own at least during the first half against modified BMW M3 and M4's, modern modified Mustangs including a GT500 but it makes me want to look into what I can do to improve the brakes and what fluid might not boil over?

[Glyn Ruck]

Hi Primaz,
Here are the basic dry & wet boiling points of brake fluid types. Latest DOT 4 LV class 6 brake fluids can achieve slightly over 300C dry. (Super DOT 4)

screen-shot-2014-12-15-at-10.27.18-am.jpg (110.74 KiB) Viewed 2648 times

The answer to your problem does not really lie in the fluid. It lies in the size of the brakes & as such their heat dissipation & cooling.

The solution to your question is to increase disc/rotor & caliper size so that they expose greater area to the air that cools them. Ventilate discs all round & duct air to the brakes.

Beyond this the answer lies in carbon ceramics etc. & exotic friction materials for the pads. Some ceramic etc. shielding of heat from the hydraulics can also help.

Brakes are all about size. Bigger disc & pad area, greater pressure & proper cooling.

Brakes turn kinetic energy into heat that must be dissipated.

[NigelW]

I was going to say basically the same thing, you are looking to treat the symptom not the cause. For this it might be an idea to look to racing as most things have been encountered before and and a solution found. One problem with high end braking systems is that what works on a race track won't work on the road for everyday driving as to make the brakes work you need to get heat into them, too cold and you will sail through the first stop light you come to. Keep us posted on what you come up with.

[Glyn Ruck]

Exactly! e.g. Formula 1 carbon brakes operate in a temperature range from approx 400 degC to 1000 degC.

Below 400 degC their retardation is pathetic. No good for a road car.