fleming23

I know the general idea is that once you open a bottle of brake fluid, you allow air to enter and it is magically no good. However, with an expensive bottle like Castrol SRF, how long is it recommended to use a bottle of previously opened fluid. For example, when bleeding brakes at the track or before a track day and you have a bottle that is 1-2 months old. Unless the cap was just left off, I can't imagine the fluid could have absorbed enough water to be no good right? Would a test strip reveal anything? If I have to do a quick bleed and only put a few ounces through the system, there is no way I am going to throw away a relatively new bottle of SRF. If it was some off the shelf cheap stuff, maybe.

Just curious, thanks!

Bill32

Before you put the cap back on, squeeze most of the air out of the plastic bottle and then put the cap on.

We bleed the calipers for every event but I've used fluid that's been in the shop all season (stored properly) and never had a problem.

speedracernc

anyone use one of these?

miracle_whip4130

Interesting, I've never used one of those.

I'm of the same thinking of the OP that if you just use a bottle of fluid once and then seal it for a couple months, it should still be ok, but don't have any hard facts to back it up other than never having my fluid boil (knock on wood).

RDnomorecobra

I don't have one but one of the instructors at an event I attended had something similar. You dip it in and it displayed the boiling point. I had him test some old fluid I had in a bleed bottle and was surprised it read something like 375 if I remember correctly. Thought it would be lower as it was very old.

ZedO6

I've used several different models of continuity brake fluid testers and they accurately display moisture content. Great for checking street car brake fluid for moisture, however I tested my track car brake fluid (ATE Gold) after an event in 98 degree temps and it still tested good, the same as the opened brake fluid I had left over from the pre track bleed.

While not a replacement for good track maintenance, it is a good tool to draw the line for "compromised" brake fluid. I tested several opened DOT 4 and in every case, they tested good. I can see the value of testing brake fluid during a track tech session. failed brake fluid does not belong on the track. Even my street cars that had brake fluid replacement during the last few months easily passed.

z28lt1

I remember reading someone on the famous oil discussion site tested a partially used bottle of Super Blue over the years and found that the boiling point had dropped (only) 5 degrees after 7 years. I suspect the metal bottle (instead of plastic) helped in that case, but that's at least a data point for reference.

johnny c

i bleed the brakes after every event a bottle only lasts a few weeks. on the street cars i use what i have around but wouldn't use anything from a container that was more them 3 months old.

fleming23

Thanks for all the replies, and about what I expected. Some who say it is fine and that the boiling point does not really change and others who will swear by changing fluid every week.

I think so long as the fluid is within a couple months from when the seal was initially cracked, I'll continue to use the old bottles. If this was a cheaper fluid, I would have no problem bleeding completely fresh fluid every track day, but part of the premises behind SRF is to not need to do this, in my opinion.

I guess this tool might be the real ticket, but I'm not going to spend the $300 on it.

redtopz

SRF is awesome stuff. I probably have bottles over a year old that I would still not hesitate to use, but I rarely have to bleed my brakes anyway. I've never had a fluid issue since the day I started using it several years ago.

ZedO6

Your input on brake fluid doesn't count, I've watched your in-car videos and you never use your brakes!

Joshboody

Semi-related question. Does everyone replace res fluid when bleeding? Or just top off after bleeding?

I do both depending on fluid age, bottles onhand, and laziness. Do you think the fluid in the line (near calipers) recirculates or mixes with res fluid?

Aardwolf

Every year I open a new bottle in summer and use till it is gone. I don't have any problem using an open bottle for a year.

The fluid at the caliper really doesn't mix much at all. The first bleed pump always is a different color than the next ones.

Once a year I bleed the reservoir out of old fluid then add new.

ZedO6

I always use a turkey baster to suck as much old fluid as I can out of the reservoir and then refill with fresh prior to bleeding. The heat up cool down cycles the reservoir fluid goes through during a track day is why it accumulates moisture and never tests out like new fluid when I test it. Why go through the effort to bleed the system with old fluid? Sucking the reservoir dry and replacing with fresh takes another 5 mins.

I think these are all good questions and you'll likely get as much variation in practices and methods as a motor oil discussion.

redtopz

Racingswh

SRF does not absorb water. If the cap is on and you didn't intentionally or otherwise add water to it somehow it's fine. It is not hygroscopic. Most other brake fluids are. If you notice SRF wet boiling point is higher than the other brake fluids available. SRF is worth it.

Hygroscopic - Wikipedia
https://en.wikipedia.org/wiki/Hygroscopy
Wikipedia
Hygroscopy is the ability of a substance to attract and hold water molecules from the surrounding environment. This is achieved through either absorption or adsorption with the absorbing or adsorbing substance becoming physically changed somewhat.

Brembo LCF600 Dry: 316°C (601°F) Wet: 204°C (399°F)

CASTROL SRF Dry: 310°C (590°F) Wet: 270°C (518°F)

MOTUL RBF600 Dry: 312°C (594°F) Wet: 216°C (421°F)

ZedO6

Castrol SRF, like other glycol or ester based brake fluids absorbs moisture. That is why Castrol calls out a wet boiling point in their product spec. Awesome wet spec considering it's still over 500 F and that's at nearly 4% by weight water!


Don't believe me?

Castrol® React SRF™ Racing Brake Fluid

Castrol SRF is the ultimate racing brake fluid.
Its unique silicon ester technology absorbs less water than conventional glycol ether fluids and prevents the fluid's high-temperature performance from deteriorating.It's high boiling point, ability to withstand extreme temperatures and superior resistance to the effects of absorbed water have established Castrol SRF Brake Fluid as the world's premier fluid for the hydraulic brakes used in all forms of motorsport and racing.

Racingswh

Thank you. I was incorrectly of the belief that it did not absorb water/moisture at all.

Racingswh

I like this description of Castrol SRF the best and it certainly clarifies for me why it works so well.

CASTROL SRF RACING BRAKE FLUID

An ultra high performance product formulated specifically to satisfy the ever increasing stresses placed upon the braking systems used in international motorsport. The exceptional performance of Castrol SRF is due to a novel silicon ester technology pioneered by Castrol and unique to the extent that it has been granted patent-protection in numerous countries throughout the world.

Safe for all non-mineral oil-based disc and drum brake systems.
Exclusive European formula!
Exceptionally high dry boiling point of 590ºF (Wet 518ºF).
Superior anti-vapor lock properties
Less hygroscopic than conventional brake fluids - it absorbs less water in a given time. Secondly, unlike conventional glycol ether fluids, Castrol SRF reacts chemically with the absorbed water to reduce its
adverse effects, thus preventing the fluid's high temperature performance and safety margins from deteriorating as rapidly as they would otherwise do.


Miscible with all conventional brake fluids conforming to these standards. However, mixing Castrol SRF and conventional brake fluids will reduce the benefits of Castrol SRF. It is
strongly recommended that conventional brake fluid be drained from the system before flushing and refilling with Castrol SRF.

FASTFATBOY

Since a refrigerator is a dehumidifier............Anyone try to keep it in there to stop it from soaking up moisture?