A flush does not remove all previous fluid.

I did some testing with a true boil point tester and found that a single flush is about 90% effective. That is to say even when doing the flush carefully and to the letter you leave some old fluid behind. The remaining fluid keeps some of the aspects of the older fluid.
This testing was done with SRF as the replacement fluid on three different 911s. I found that it took 2 flushes with some driving between each flush to blend the old with the new to get the boil point to come up to full SRF spec.

The type of fluid to use depends on the car use. Live in a humid climate and drive hard? SRF has a very high wet boil point and even when old offers higher resistance to boiling than others. If you have to pay labor to flush the brake fluid a high wet boil point fluid might be less expensive in the long run as you can go longer between flushes. The $70 for the SRF might be offset by going 2X as long between $150 labor costs.

Race or drive hard and are a nut about fluid service? Motul will give you the same as SRF for a fraction of the price.

Don't do hard drives and are a nut about fluid service? Any of the better fluids will work well.

Live in the desert? Don't worry about the wet point too much.


YMMV

All in all, good points. I also will sometimes use the cheap stuff for a major flush, and then go back to ATE Blue or Gold for the final fill.

I am skeptical of the 90% claim. I mean, if you really want to get academic, then one could say that you will never get to the 100% level, regardless of what you do. I mean, you would need to begin with "new" good fluid, which would then be superceded by the "newer" fluid, again at 90%......and so on.

Jeez, don't even get me started on oil changes...:eek:

Cheap or not, I really like your idea.

If you want good and cheap, try Valvoline Synthetic brake fluid.

Thanks for all the great posts! ATE seems to be the fluid of choice here so maybe I'll use the Castrol for the first "cheap" flush.
I have heard the Motive bleeder is a good one one but a few sources recommend a maual pedal bleed after they use the power bleeder. Do you guys think that is necessary?

Scot

Ate Super Blue/Type #200 for street cars and normal track use

Motul RBF600 for many race cars

Castrol SRF for race cars driven aggressively and for cars with small brakes

Yeah I know. It's not old fluid, it's cheap new fluid. I'm ok with a 90% mix of cheap stuff. I'm a cheap bastich.

OK lets leave numbers out and discus findings in general.
I found that after one flush the fluid in the system, after some driving to mix, was always between the new fluid and the old fluid. It was never at the new fluids rating. The lower the boil point of the old fluid the greater the delta. After the second flush the numbers were still below pure fresh but close enough to the new fluids dry rating to be functionally the same. Single digit deltas. My findings lead me to believe that when you flush old low boil point saturated or lesser fluid you leave some behind. For 99.5 % of street cars this matters not one bit. For race cars that are driven at the limits of the brake system this might mean the difference if the old stuff was in poor condition. Fresh to what DOT considers saturated and the wet point is 3% water absorption. I have seen fluid come out of street cars with rust and droplets of water in it. (it tested at 211 deg at 300 feet from sea level)

Another thing people do with fluid is toss partly used metal cans.
My buddies at the track give me metal cans of fluid that they opened 3 or 6 or whatever months ago and are discarding. This is due to popular opinion that fluid starts going bad in the can the second you open it. I test the fluid from given cans and have always found it to be at or a deg or 3 from the new dry rating. I have tested the fluid right from freshly opened cans and always found it to be at the dry number within the testing accuracy limits of the tester. I have a can of ate blue that is 1/2 full, opened many years ago, stored in an outside shed in humid summers and tests to within 8 deg of fresh.
With that said I will never use fluid from an all plastic container. From limited testing I found that fluid in plastic containers does absorb moisture. I have a plastic container of DOT3 FLAPS slop that is heading down with time.

Boil point testers are not too expensive. They remove question regarding fluids and methods.

Another vote for Castrol GTLMA!

It takes several flushes and a lot of use to get all traces of the previous fluid out, I found this out when I moved away from that ATE Blue "stains everything it touches" crap. For a street car I would use as a minimum ATE Gold, then Motul 600, then Motul 660, then Castrol SRF (best fluid out there). The Castrol seems super expensive until you realize you get twice the fluid as all the others, then it just seems expensive. :D

I run Motul 660 and it's around $25 a pint ($10 higher than ATE) vs the Castrol SRF at $35 per pint.. I do a quick flush about 3 times per year in my race car and once per year in my street cars.

Green 912

a) your first post came through fine. Did your cat step on your keyboard after that?

b) but it is such a good post it was worth reading three times!

What does a brake fluid boiling temp tester cost? I vaguely remember a catalog seller selling a water in the brake fluid tester, but at a price which was more than I was inclined to pay.

I like the idea that I can use the old new fluid from a can. Which is what I do anyway.

I don't like the idea that I am taking more of a risk when I do that with my expensive Castrol SRF, which now comes in a plastic container.

I think that for street use, just about any fluid is probably OK for an SC. I got along fine with the Castrol LMA stuff. You shouldn't be heating your brakes up while driving on public roads, and a one time really heavy application isn't going to boil fluid.

But my 911 SC track/race car fits Steve's "driven hard with small brakes" category. Cool brake air scoops and ATE fluid weren't quite enough. To make it work as a 2 driver track car I had to: duct air from the air dam to the brakes; install the stuff to blow that air into the center of the rotor, and block its escape out the other side; install titanium insulating plates - thin plates of a metal which conducts heat poorly/insulates well that slip between the piston and the pad backing plate; use stainless steel pistons; and use the SRF, which at the time was about the highest dry boiling point fluid around. It is still the highest wet boiling point, I think. With that setup I never have to bleed the brakes.

But I am experimenting to see if the Motul will work.

My track only car is lighter, and has 4 piston calipers all the way around. Air ducted to the fronts only. I have had fine results from the ATEs two colors, and sometimes have gone several years without bleeding, to say nothing of changing fluid.

So it depends. But going the ATE route (is there any other ATE than the blue and its twin gold?) is a pretty safe bet on average.

No cat, just fat fingers. Double post fixed.

I am stopping a 73 RSR sort of look alike track car with S in front and M in back and stock rotors. It is (very well) propelled by a 1995 3.6 motor. No issues on track with cooking the brakes. I have huge brake air intakes in front. Using ATE from given to me cans.

Testers are now running in the $300 range so it will take a lot of free 1/2 full cans to make your money back. I purchased mine after seeing a lot of conjecture and repeated "truths" when i purchased it I the Tech Chair for a PCA region and was getting a lot of "Sure I changed my brake fluid just last week, yea that's it. My formula 1 mechanic changed it while I was out on a hot date with a super model, yea that's the ticket." I would look at it and it would be old and cloudy. I did a lot of flush demonstrations and free fluid testing to show how bad fluid could get in street cars. It gets real bad in the humid mid atlantic in short order.

BTW. You never get push back about brake fluid from an experienced track driver.

Re, plastic container and SRF. SRF is a bit different in that its wet and dry points are both high and not too far apart. I don't yet have a 1/2 bottle of SRF to shelve and see if it degrades in the plastic container like the cheap stuff does.

Higher boiling point, less moisture absorption,...All around better characteristics for brake fluid, IMHO.

Wurth has a very nice little electronic fluid tester, ~$100, there are other similar generics and some much cheaper alternatives using test strips.

I'll third or fourth (or whatever) Motul RBF600. I've used ATE SuperBlue and the stock Porsche brake fluid, but have found Motul RBF600 to be the best.

I'm not sure re the live in the desert comment - not a practical issue - just change it, but I'm wondering what the absolute humidity corresponds to for each level of fluid degradation from moisture content...

i.e. need to go from g/m^3 to % in the brake fluid

Good stuff but, along with what type, frequency of changing fluid and what use like street or track should be part of the issue.

All brake fluids have some H20 or attract some over time.
Regular flushing is as important as what type.
Exercising the pistons will prevent rusting/seizing.
Cooling the calipers/rotors will greatly help, i.e. WA, OR, BC climate keeps them cooler than CA, AZ, UT.

Every time I remove the wheels, I carefully push back the pads/pistons and use the pedal to bring them back, repeat 3 times. No problems ever with seizing. :cool:

Some have posted that this should'nt be done because it may move the pistons into areas of the bore that may contain dirt/debris. That makes no sense because when one installs new pads, the pistons have to be pushed back all the way. :rolleyes:

I have exercised the pistons with different cars/systems for many years and recommend doing it plus change the fluid regularly; DOT 3 for street and better stuff for track. :)

Good point Gunter, and when installing new or rebuilt calipers it's almost a necessity to exercise them 3-5 times throughout their range to get every bit of air out. I keep an old set of pads just for this using my spreader tool.

Yes, an old set of pads for sure so the pistons don't pop out. :)

I found that pushing the pads/pistons back is easiest with large channel-pliers gripping the steel-portion of the pads and the caliper-body and then squeeze.

Endless RF 650 in all the race cars and Castrol GT LMA for street...You'll not find a better fluid than Endless IMHO...