Changing brake fluid from SRF to a Dot4 / 5.1

Non-engine, eg: aerodynamics, gearboxes, brakes, suspension
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mtbr
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Changing brake fluid from SRF to a Dot4 / 5.1

Post by mtbr »

When changing from Silicon brake fluid to a conventional Dot fluid, is it necessary to flush the system?
If so what flushing fluid is required.

Many thanks, Mark
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Guy Croft
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Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by Guy Croft »

No that is not necessary.

It is advised to strip the system and thoroughly clean if changing from glycol ether fluids (that absorb water) to silicone fluid - but not vice-versa.

FWIW for those not familiar with these porducts silicone fluid doesn't attack paintwork and doesn't absorb water (well, it absorbs a tiny proportion, but compared to glycol types effectively none). Those are the only advantages; mostly only classic car owners use since their cars can be sat around for long periods out-of-use. I have read some VERY mixed reports about the effect of silicone fluid on brake system rubber seals and brake fade etc etc., so anyone who's racing (considering this is a race website) is well advised to get the tech literature from the supplier (I recommend AP Racing because they do at least publish stuff) before buying. The DOT code on brake fluid refers to the temperature rating of the fluid BTW. I think we're up to 5 or so these days. Read about AP and their remarks esp on silicone at:

http://www.apracing.com/info/index.asp? ... +Fluid_978


GC
mtbr
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Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by mtbr »

I spoke to AP Racing's technical help this morning, their recommendations were:

1. Flush with the new fluid you intend to use.
2. Push the pistons in the calipers back to clear the calipers.
3. Allow for up to 4 complete flushes.
4. It is not necessary to dismantle the system.
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Guy Croft
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Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by Guy Croft »

No-one in their right mind would argue with AP Racing over that.

I assumed that since you had silicone fluid in there already it wasn't a competition car, and thus small residual amounts of silicone fluid would not be detrimental to the kind of braking you're used to. Nothing wrong of course with getting rid of all of it - which is what they have described. It's noteworthy that AP Racing don't recommend the use of silicone fluid at all, thus they would naturally want to see it all gone before adding one of their proven products.

GC
Evodelta

Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by Evodelta »

SRF mixes fine with conventional types, so no major and extensive flushing activities are needed.

SRF is acknowledged as The best and this is the 'Competition car' section so I am intigued as to why you would be downgrading?
mtbr
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Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by mtbr »

I have struggled to achieve a solid brake pedal with SRF. I have also found SRF needs changing far more frequently than other conventional racing brake fluids. The brake temperatures I am achieving with my car (estimated using temperature sensitive paint and cross checked with a non contact IR thermometer ) are well within the operating range for a number of high performance conventional fluids. In this case, it is not really a downgrade. (I also hope to free up some "preparation" time and save a little money, both of which are needed for other areas on the car.)

The issue of feel is quite interesting.
The pedal has never been spongey, as it is with air in the system. In fact, if anything, it felt like the pedal box was flexing. I even put a small video camera in the footwell at one point. The only movement on the video was the pedal itself. Other examples of the same car (Lotus Elise) I've driven, have not exhibited this "give" in the brake pedal either. These cars have a simple braking system without a servo and it's normally quite easy to achieve a very solid brake pedal with almost no movement at all.

I have seen unsubstantiated claims that SRF is more compressible than some other brake fluids, I must say I find this suggestion hard to believe, however I have not experienced this problem before with Elises and the only thing that is different in this case is the brake fluid. I will post again after I have tested the car with DOT4 fluid.

Mark
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Evodelta

Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by Evodelta »

That's interesting about the spongyness (or similar, as you say) of the pedal Mark, I've read only various 'forum banter' about this and just assumed they had not bled the brakes properly. I never noticed it myself, but then I don't think I am that sensitive to minor changes on a car and my integrale is far heavier and harder on its brakes than an Elise therefore I really do think I need it.

Other forum bull I have read is the life of SRF, many say it must be changed at 12 months, this is not so, Castrol recomend 18months - 6 months shy of normal Glycol fluids and I have stretched it further than that to 2yrs with no ill effects. I like your open mindedness and would like to hear more about your results and conclusions.

Martin.
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Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by FIAT125T »

From my reasearch on the interweb it seems that the major problem with Silicone type brake fluids is that they do not absorb water. Any air in the system which always has moisture in it, creates pockets which will boil and reduce braking efficiancy dramatically. Normal fluid absorbs this water and loses efficiancy much more gradually. This only really applies to racing situations where sustained high temps are encountered. All Silicone fluids are Dot 5.0 and dont make the mistake of confusing Silcone 5.0 with Synthetic 4 and 5.1 which is fully compatible with trad Glycol applications.
Keeper of a 125T the flyin' New Zealander Fiat.
Mats
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Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by Mats »

Silicone brake fluids were developed to overcome performance issues with ABS systems in very low temperatures, not really an issue for the normal track day car or race car.
The DOT rating includes an index of how hygroscopic the fluid is and this is why normally a race fluid performs worse (in the ratings), the manufacturers aim for performance rather then long service life. The problem with this rating system is that the real performance is kind of lost, most race fluids (even though only rated DOT3 or DOT4) like for instance Castrol SRF or Motul RBF have a much higher wet boiling point then most DOT5.1 fluids dry boiling point...
See my point?
/Mats Strandberg
mtbr
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Re: Changing brake fluid from SRF to a Dot4 / 5.1

Post by mtbr »

This afternoon I was able to test my car with a substantially revised braking system fitted. The results have surpassed all my expectations. Due in the main to a lamentable lack of patience (and free time), I have completely disregarded the first principle of development work, which is to only make one change at a time.
Instead, in the last two days, I have carried out the following modifications:

1. Replaced the master cylinder, There was some light scoring evident in the old one.
2. Fitted new "AP Racing" rotors x 4 (on custom made aluminum mounting bells).
3. Overhauled both rear calipers, cleaned and inspected the front ones.
4. Replaced the Castrol SRF silicon brake fluid with AP "PRF 660" fluid.

I have now achieved the pedal feel I was looking for. During today's test, there was no trace of the previous springiness and the brakes were easy to modulate at or close to maximum braking effort.

As a result of the somewhat unscientific process I have followed, I can't conclusively state that the silicon fluid was responsible for the lack of precision and feel I struggled with last season. In particular the defects in the old master cylinder may have been a contributory factor, however I can say this problem is now solved.
This test was carried out at North Weald aerodrome, using part of the B19 sprint course, in spite of completing several blocks of 6 laps, the brakes never got really hot, however from previous data, I don't expect any heat related issues with this new brake fluid.

I will be running the car at Zandvoort this weekend and at Dijon on Monday, both these circuits will expose any brake cooling issues. I will also have the data logger working for these events, so hopefully I will have some real data to share next week.

regards, Mark
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